Chemistry of Opioids

299 Topics in Current Chemistry Editorial Board: A. de Meijere K.N. Houk C.A. Hunter H. Kessler J.-M. Lehn S.V. Ley M...

Author: Hiroshi Nagase

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299

Topics in Current Chemistry

Editorial Board: A. de Meijere K.N. Houk C.A. Hunter H. Kessler J.-M. Lehn S.V. Ley M. Olivucci J. Thiem B.M. Trost M. Venturi P. Vogel C.-H. Wong H. Wong H. Yamamoto l

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Chemistry of Opioids

Volume Editor: Hiroshi Nagase

With Contributions by S.N. Calderon N. Chida H. Fujii S. Hirono S.M. Husbands J.W. Lewis K.M. Lovell A. Lozama H. Nagase K.M. Prevatt-Smith T.E. Prisinzano H. Schmidhammer M. Spetea N. Yamaotsu

Editor Prof. Dr. Hiroshi Nagase Kitasato University School of Pharmaceutical Sciences Department of Medicinal Chemistry Sirokane 5-9-1 108-8641 Tokyo Minato-ku Japan [email protected]

ISSN 0340-1022 e-ISSN 1436-5049 ISBN 978-3-642-18106-1 e-ISBN 978-3-642-18107-8 DOI 10.1007/978-3-642-18107-8 Springer Heidelberg Dordrecht London New York # Springer-Verlag Berlin Heidelberg 2011 This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilm or in any other way, and storage in data banks. Duplication of this publication or parts thereof is permitted only under the provisions of the German Copyright Law of September 9, 1965, in its current version, and permission for use must always be obtained from Springer. Violations are liable to prosecution under the German Copyright Law. The use of general descriptive names, registered names, trademarks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. Cover design: WMXDesign GmbH, Heidelberg, Germany Printed on acid-free paper Springer is part of Springer Science+Business Media (www.springer.com)

Volume Editor Prof. Dr. Hiroshi Nagase Kitasato University School of Pharmaceutical Sciences Department of Medicinal Chemistry Sirokane 5-9-1 108-8641 Tokyo Minato-ku Japan [email protected]

Editorial Board Prof. Dr. Armin de Meijere

Prof. Dr. Jean-Marie Lehn

Institut fu¨r Organische Chemie der Georg-August-Universita¨t Tammanstr. 2 37077 Go¨ttingen, Germany [email protected]

ISIS 8, alleé Gaspard Monge BP 70028 67083 Strasbourg Cedex, France [email protected]

Prof. Dr. Kendall N. Houk

Prof. Dr. Steven V. Ley

University of California Department of Chemistry and Biochemistry 405 Hilgard Avenue Los Angeles, CA 90024-1589, USA [email protected]

University Chemical Laboratory Lensfield Road Cambridge CB2 1EW Great Britain [email protected]

Prof. Dr. Christopher A. Hunter

Prof. Dr. Massimo Olivucci

Department of Chemistry University of Sheffield Sheffield S3 7HF, United Kingdom [email protected]

Universita` di Siena Dipartimento di Chimica Via A De Gasperi 2 53100 Siena, Italy [email protected]

Prof. Dr. Horst Kessler Institut fu¨r Organische Chemie TU Mu¨nchen Lichtenbergstraße 4 86747 Garching, Germany [email protected]

Prof. Dr. Joachim Thiem Institut fu¨r Organische Chemie Universita¨t Hamburg Martin-Luther-King-Platz 6 20146 Hamburg, Germany [email protected]

vi

Editorial Board

Prof. Dr. Barry M. Trost

Prof. Dr. Chi-Huey Wong

Department of Chemistry Stanford University Stanford, CA 94305-5080, USA [email protected]

Scipps Research Institute 128 Academia Road, Section 2 Nankang, Taipei Taiwan [email protected]

Prof. Dr. Margherita Venturi Dipartimento di Chimica Universita` di Bologna via Selmi 2 40126 Bologna, Italy [email protected]

Prof. Dr. Henry Wong The Chinese University of Hong Kong University Science Centre Department of Chemistry Shatin, New Territories [email protected]

Prof. Dr. Pierre Vogel Laboratory of Glycochemistry and Asymmetric Synthesis EPFL – Ecole polytechnique fe´derale de Lausanne EPFL SB ISIC LGSA BCH 5307 (Bat.BCH) 1015 Lausanne, Switzerland [email protected]

Prof. Dr. Hisashi Yamamoto Arthur Holly Compton Distinguished Professor Department of Chemistry The University of Chicago 5735 South Ellis Avenue Chicago, IL 60637 773-702-5059 USA [email protected]

Topics in Current Chemistry Also Available Electronically

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Aims and Scope The series Topics in Current Chemistry presents critical reviews of the present and future trends in modern chemical research. The scope includes all areas of chemical science, including the interfaces with related disciplines such as biology, medicine, and materials science. The objective of each thematic volume is to give the non-specialist reader, whether at the university or in industry, a comprehensive overview of an area where new insights of interest to a larger scientific audience are emerging.

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Topics in Current Chemistry Also Available Electronically

Thus each review within the volume critically surveys one aspect of that topic and places it within the context of the volume as a whole. The most significant developments of the last 5–10 years are presented, using selected examples to illustrate the principles discussed. A description of the laboratory procedures involved is often useful to the reader. The coverage is not exhaustive in data, but rather conceptual, concentrating on the methodological thinking that will allow the nonspecialist reader to understand the information presented. Discussion of possible future research directions in the area is welcome. Review articles for the individual volumes are invited by the volume editors. In references Topics in Current Chemistry is abbreviated Top Curr Chem and is cited as a journal. Impact Factor 2009: 4.291; Section “Chemistry, Multidisciplinary”: Rank 20 of 138

Preface

As part of the Topics in Current Chemistry series, this volume, entitled “Chemistry of Opioids,” presents the progress made in opioid research over the past 5 years in the synthesis and pharmacology of opioids. In 1803, morphine was first isolated by a German pharmacologist, Sertu¨rner, and its structure was later determined. Since then, many research groups have tried to synthesize an ideal analgesic without addictive properties. In the course of those studies, a number of opioids were synthesized. Morphine has a unique 4,5-epoxymorphinan skeleton, which comprises five sequential asymmetric centers and a phenolic hydroxy group, a 4,5-epoxy ring, and a basic nitrogen. Due to this complex structure, early research groups could not synthesize morphine. Therefore, they tried to design drugs with simpler structures that could be manufactured as an industrial product. These trials produced many drugs, including pentazocine (benzomophan), butorphanol, levorphanol (morphinan), pethidine (phenylpiperidine), fentanyl (anilinopiperidine), etc. However, these compounds could not provide analgesics without addiction. In 1975, when endogenous opioids were isolated, many researchers expected to be able to produce a drug without addictive properties, and many peptide derivatives were synthesized. Unfortunately, these opioid peptide mimetics also had addictive properties, despite the fact that they were derived from endogenous substances. Nevertheless, opioid researchers did not give up their dreams. The first clue was that there were three types of opioid receptors (m, d, k). This explained the diverse pharmacological effects of the many synthetic drugs. Many of the syntheses and pharmacologies of drugs produced before the early 1980s have been described in detail in two excellent books published in 1986, entitled Opioid Analgesics; Chemistry and Receptor and Opiates. After that, research focused on the k and d opioid receptor types. In the 1990s, extensive research was conducted in multiple efforts to synthesize k selective agonists. However, there is no current book regarding the progress in the whole opioid field of d- and k-type selective ligands. In particular, in the 1990s, a vast number of U-50,488H (discovered by Upjohn Company) derivatives were synthesized to produce a nonaddictive drug. Unfortunately, these derivatives showed aversive effects (psychotomimetics), and development was suspended in the early stages, both as an analgesic and as an antipruritic ix

x

Preface

drug. Consequently, there have been no reports on U-50,488H derivatives in the last 5 years. This book describes the progress made in the opioid field over the last 5 years with a focus on the work of 14 representative researchers in the opioid field. The first chapter, “Recent Advances in the Synthesis of Morphine and Related Alkaloids,” by Noritaka Chida, presents the racemic synthesis of morphine and related alkaloids by four research groups, and the chiral synthesis of those alkaloids by four research groups. This work has greatly contributed to progress in Organic Chemistry and Medicinal Chemistry. The second chapter, “Opioids in Preclinical and Clinical Trials,” by Hiroshi Nagase and Hideaki Fujii, provides a short survey of opioid ligands in development. They also give a detailed history of the drugs, TRK-851(d-antagonist) and TRK820 (nalfurafine hydrochloride, k-agonist), which were discovered by their team. The third chapter, “Synthesis of 14-Alkoxymorphinan Derivatives and Their Pharmacological Actions,” by Helmut Schmidhammer and Mariana Spetea, presents recent advances in the chemistry, ligand-based structure activity relationships, and pharmacology of 14-alkoxymorphinans. The fourth chapter, “14-Amino-4,5-epoxymorphinan Derivatives and Their Pharmacological Actions,” by John W. Lewis and Stephen M. Husbands, presents the recent extensive studies on the chemistry, structure–activity relationships, and pharmacology of 14-acylamino- and 14-alkylamino-derivatives. The fifth chapter, “Nonpeptidic d Opioid Agonists and Antagonists of the Diarylmethylpiperazine Class: What Have We Learned?” by Silvia N. Calderon, presents the major advances in the field of d-opioid ligands and the extensive research performed to uncover the SAR of SNC-80 derivatives. Furthermore, synthetic methods are described for these compounds. The sixth chapter, “Synthesis of Neoclerodane Diterpenes and Their Pharmacological Effects,” by Kimberly M. Lovell, Katherine M. Prevatt-Smith, Anthony Lozama, and Thomas E. Prisinzano, describes the total synthesis of Salvinorin A and current research efforts focused on structure modifications of Salvinolin A derivatives. The seventh chapter, “Synthesis of Novel Basic Skeletons Derived from Naltrexone,” by Hiroshi Nagase and Hideaki Fujii, describes the many novel reactions that were discovered in the course of synthesizing naltrexone derivatives and the characteristics of those naltrexone derivatives. Some of the new reactions were expanded into general reactions. The eighth chapter, “Twin and Triplet Drugs in Opioid Research,” by Hideaki Fujii, presents opioid research that used dimer and trimer ligands as a tool for investigating opioid receptors. The dimer and trimer constructs were able to increase ligand activity and selectivity. The ninth chapter, “3D-Pharmacophore Identification for k-Opioid Agonists Using Ligand-Based Drug-Design Techniques,” by Noriyuki Yamaotsu and Shuichi Hirono, presents a summary of previous efforts in identifying the pharmacophore in k-opioid agonists and a proposal for a new model that encompasses the activities of all class k-agonists.

Preface

xi

I would like to thank all those who have contributed to making this volume a collection of expert articles. I hope that our readers find this volume a useful guide to the current progress in opioid research. Tokyo, Summer 2010

Hiroshi Nagase

.

Contents

Recent Advances in the Synthesis of Morphine and Related Alkaloids . . . . 1 Noritaka Chida Opioids in Preclinical and Clinical Trials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 Hiroshi Nagase and Hideaki Fujii Synthesis of 14-Alkoxymorphinan Derivatives and Their Pharmacological Actions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 Helmut Schmidhammer and Mariana Spetea 14-Amino-4,5-Epoxymorphinan Derivatives and Their Pharmacological Actions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 John W. Lewis and Stephen M. Husbands Nonpeptidic Delta (d) Opioid Agonists and Antagonists of the Diarylmethylpiperazine Class: What Have We Learned? . . . . . . . . . . . . . . . . 121 Silvia N. Calderon Synthesis of Neoclerodane Diterpenes and Their Pharmacological Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Kimberly M. Lovell, Katherine M. Prevatt-Smith, Anthony Lozama, and Thomas E. Prisinzano

141

Synthesis of Novel Basic Skeletons Derived from Naltrexone . . . . . . . . . . . . 187 Hiroshi Nagase and Hideaki Fujii Twin and Triplet Drugs in Opioid Research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239 Hideaki Fujii 3D-Pharmacophore Identification for k-Opioid Agonists Using Ligand-Based Drug-Design Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 277 Noriyuki Yamaotsu and Shuichi Hirono Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 309 xiii

.

Top Curr Chem (2011) 299: 1–28 DOI: 10.1007/128_2010_73 # Springer-Verlag Berlin Heidelberg 2010 Published online: 10 June 2010

Recent Advances in the Synthesis of Morphine and Related Alkaloids Noritaka Chida

Abstract Morphine, an alkaloid isolated from the opium poppy, has been widely used as an analgesic, and has been a fascinating synthetic target of organic chemists. After the first total synthesis reported in 1952, a number of synthetic studies toward morphine have been reported, and findings obtained in such studies have greatly contributed to the progress of synthetic organic chemistry as well as medicinal chemistry. This review provides an overview of recent studies toward the total synthesis of morphine and related alkaloids. Work reported in the literature since 2004 will be reviewed. Keywords Chiral synthesis Codeine Morphine Total synthesis Contents 1 2

Introductory Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Synthesis of Morphine and Related Alkaloids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.1 Racemic Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.2 Chiral Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 3 Closing Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Abbreviations aq BHT Cy

Aqueous 2,6-Di-tert-butyl4-methylphenol Cyclohexyl

N. Chida (*) Department of Applied Chemistry, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan e-mail: [email protected]

2

N. Chida

dba DIAD DMP dppe dppf dppp MS3A o-tolyl Troc

Dibenzylideneacetone Diisopropyl azodicarboxylate Dess–Martin periodinane 1,2-Bis(diphenylphosphino)ethane 1,10 -Bis(diphenylphosphino)ferrocene 1,3-Bis(diphenylphosphino)propane Molecular sieves 3A 2-Methylphenyl 2,2,2-Trichloroethoxycarbonyl

1 Introductory Remarks More than two hundred years have passed since the isolation of morphine (1) from the unripe seed pods of opium poppy, Papaver somniferum [1, 2]. A number of structurally related alkaloids, such as codeine (2), thebaine (3), and codeinone (4), have also been discovered in the same plant (Scheme 1) [3]. Due to its significant effectiveness as an analgesic and as an anesthetic, morphine has been used as an important medicine for a long time in spite of its serious addictive side effects. The structure of morphine, first proposed in 1925, is quite unique among alkaloids; it has a strained pentacyclic core (morphine skeleton) with five contiguous chiral centers including a benzylic quaternary carbon [4]. The significant biological activity as well as the highly challenging structure of morphine has naturally received considerable attention from synthetic chemists, and thus a huge number of synthetic HO

MeO

MeO

O

O

O H

NMe

H

MeO

HO

HO 1

NMe

NMe 3

2

2

MeO

3

NMe HO

4

O O HO

H

NMe

E

O 5 6

O

A

1 11 15

12 13

C

10

B 16

14 9

H

D

NR

8

7

4

morphine skeleton and atom numbering

Scheme 1 Structures of morphine, codeine, thebaine, codeinone, and a morphine skeleton

Recent Advances in the Synthesis of Morphine and Related Alkaloids

3

studies and total syntheses have been reported after the first total synthesis by Gates and Tschudi in 1952 [5, 6]. During the extensive synthetic studies of morphine, many useful new reactions and innovative synthetic methodologies have been developed, bringing significant progress in the field of organic chemistry [7–9]. The developments of the novel and effective synthetic method of morphine and its related compounds are still an important task in modern synthetic organic chemistry. In this chapter we will deal with the recent synthetic studies of morphine and its related alkaloids in the literature since 2004, as the excellent and comprehensive reviews covering the literature prior to 2004 by Maier [7], Multzer [10], Blakemore and White [11], Hudlicky [12, 13], Taber [14], and Dalton [15] have been published.

2 Synthesis of Morphine and Related Alkaloids Although a huge number of synthetic studies toward the total synthesis of morphine and related alkaloids have been reported to date, it is still important to establish the novel and efficient synthetic pathways to these alkaloids. In particular, developments of flexible synthetic methods, which can produce morphine analogs that are difficult to obtain by the chemical modification of natural alkaloids, are highly important for the generation of a novel morphine-type drug showing lower or no addictive effect. From the viewpoint of organic synthesis, the complex pentacyclic structure of morphine with multi stereogenic centers remains a challenging target. In this chapter, recent literature describing the total synthesis of morphine and related alkaloids reported from eight research groups during 2004–2009 will be reviewed. They are classified into two categories: racemic synthesis and chiral synthesis.

2.1

Racemic Synthesis

In this section, reports on the total synthesis of morphine-related compounds in racemic forms based on the novel and synthetically important methodologies disclosed by Fukuyama, Guillou, Stork, and Magnus will be discussed.

2.1.1

Fukuyama Group

In 2006 Fukuyama published a total synthesis of racemic morphine starting from isovanillin and a cyclohexene-epoxide [16, 17]. The key features in their synthesis are (1) a construction of the ether linkage between A and C rings by Tsuji-Trost coupling, (2) an intramolecular Heck reaction to construct A–C–E tricyclic system, and (3) an intramolecular Mannich-type reaction of a ketone with an aminal to provide the pentacyclic structure of morphine in a one-step reaction by double cyclization.

4

N. Chida OTBS

MeO

1) AcOH aq THF OMe

MOMO

OMe

I 5

MeO

2) MeOCH2PPh3Cl NaHMDS, THF 3) HCl, MeOH (81%)

O 7

OMe

HO

OMe

MeO

MeO OMe

OMe

MeO

MeO O

Pd2(dba)3 P(2-furyl)3 CH3CN (91%)

I 6

1) p-nitrobenzoic acid PPh3, DEAD

I OTBS

O

PPh3, DEAD toluene

I OH

2) CSA, MeOH

2) LiBH4 Et2O/MeOH 3) TBSCl imidazole DMF (91%)

(85%) OH

OBz(p-NO2)

8

9 DIBALH CH2Cl2 –78 °C then NaBH4 MeOH

MeO OMe MeO O

I

MeO OMe MeO O

Pd2(dba)3 P(o-tolyl)3

I

then ClCO2Me K2CO3

CN

HO CN

1)

NHCO2Me

Et3N CH3CN

(94%) OTBS 11

OTBS 10 MeO

MeO

OMe OMe

O

NHCO2Me

(87% for 2 steps)

OMe

O E

NHCO2Me

C O

OTBS 12

OMe

A TBAF

13

Scheme 2 Construction of the A–C–E ring of morphine by Fukuyama

Their synthesis begins with the known aryliodide 5 (Scheme 2). After one-carbon homologation of 5, the resulting phenol 6 was treated under the Tsuji-Trost conditions [18] with cyclohexene-epoxide 7 to afford aryl ether 8 with complete regioand stereoselectivity. After inversion of the stereochemistry of the secondary alcohol function in 8 by Mitsunobu reaction [19, 20] with p-nitrobenzoic acid, the primary hydroxy group in 9, obtained by the deprotection of TBS group, was converted into a nitrile under the Mitsunobu conditions. Exchange of the protecting

Recent Advances in the Synthesis of Morphine and Related Alkaloids MeO

MeO

MeO

5

OMe OMe

O

B HCl

NCO2Me OMe (94%) O

NHCO2Me MeOH reflux O 13

D

O

O

14

H O

NCO2Me

15

Scheme 3 Construction of D- and B-rings by way of the double cyclization via an intramolecular Mannich-type reaction

group of the hydroxy group afforded 10. The nitrile function in 10 was reduced with DIBALH to give a crude imine, which was further reduced with NaBH4 in methanol to give a primary amine. Protection of the amine as a methyl carbamate provided 11. The intramolecular Heck reaction to construct the polycyclic benzofuran skeleton in morphine was reported by Overman [21] in their synthesis of (þ)- and (–)dihydroisocodeine in 1993, and after that, the Heck cyclization has been extensively studied by Cheng [22], Hsin [23, 24], Hudlicky [25] (see Sect. 2.2.3), Trost (see Sect. 2.2.1), and Guillou (see Sect. 2.1.2). In Fukuyama’s synthesis, the Heck reaction of 11 works well and successfully provided the A–C–E tricyclic ring system 13 in 87% yield after deprotection of the TBS group in the intermediary enol ether 12. When acetal 13 was treated with methanolic HCl at reflux temperature, double cyclization took place to give morphine skeleton 15 in 94% yield (Scheme 3). Since the formation of eight-membered aminal 14 was observed during the reaction, this intriguing cyclization is believed to proceed via an intramolecular Mannich-type reaction. It is important to note that the morphine skeleton was effectively constructed in high yield under simple reaction conditions. For a conversion of 15 to morphine, compound 15 was first oxidized under Ito-Saegusa conditions [26] to give cyclohexenone derivative 16 (Scheme 4). After introduction of an epoxide ring, the ketone carbonyl in 17 was stereoselectively reduced to afford 18. The rigid pentacyclic structure of a morphine skeleton allowed the approach of the reagent only from the less hindered exo (b-) face. The hydroxy group in 18 was converted into a thiocarbamate and tin-mediated radical conditions [27] induced the epoxide opening to provide allylic alcohol 19. Oxidation of the alcohol function gave a cyclohexenone, which was treated with LiAlH4. Under the reaction conditions, the ketone carbonyl was reduced stereoselectively and the methyl carbamate was also reduced to give codeine. Finally, by Rice’s procedure [28], codeine was successfully converted into morphine.

2.1.2

Guillou Group

In 2008 the Guillou group disclosed the total synthesis of racemic codeine [29]. The key features in their synthesis are (1) a construction of the C-ring including a quaternary carbon by the intramolecular Heck reaction, (2) an oxy-Michael

6

N. Chida

MeO

MeO

1) TMSCl LiHMDS THF, 0 °C

O

aq H2O2 NaOH

O

15

H O

NCO2Me

2) Pd(OAc)2 CH3CN

H O

(92%)

CH3CN

NCO2Me

(91%)

16

MeO

MeO 1)

O

O

NCO2Me

MeOH/ CH2Cl2

(91%)

NCO2Me H OH

O

18

17 MeO

MeO 1) DMP CH2Cl2

O

NCO2Me 2) LiAlH4 H THF

HO 19

(78%)

N

O

NaBH4 H O

S N C N

N

(ClCH2)2 2) Bu3SnH Et3B (48%) THF

HO BBr3

O H

NMe

(74%)

HO codeine

O

CH2Cl2

H

NMe

HO morphine

Scheme 4 Synthesis of morphine by Fukuyama

addition of phenol to cyclohexadienone to generate the A–C–E tricyclic system of morohine skeleton, (3) an Eschenmoser Claisen rearrangement for the stereoselective construction of a tertiary carbon at C-14, (4) a B-ring formation by intramolecular FriedelCrafts-type cyclization/dehydration, and (5) a reductive hydroamination to make up the D-ring. Condensation of carboxylic acid 20 with 2-iodo-6-methoxyphenol gave ester 21 (Scheme 5). When 21 was treated with Pd catalyst and Et3N in DMF, the intramolecular Heck reaction successfully took place to provide spiro-compound 22. It is interesting to note that, in this reaction, the absence of phosphine ligands is essential for the completion of the reaction in shorter reaction time. After hydrolysis of the dioxolane group, the resulting ketone was converted into dienone 23 by the oxidation with (PhSeO)2O. Treatment of 23 with N-methylbenzylamine induced the lactone opening and the formation of an amide. Under the reaction conditions, an intramolecular oxy-Michael addition of the resulting phenol 24 to the dienone moiety occurred to afford the tricyclic benzofuran (A–C–E ring system) 25. Reaction of 25 with LiAlH4 stereoselectively reduced the enone system in a 1,2-fashion as well as the amide carbonyl to give 26. Eschenmoser Claisen rearrangement [30] of 26 afforded the desired rearranged product 27 in 49% yield. The major side product in the rearrangement was diene 28 (32%), and it was found that


I

MeO

CO2H O

OH OMe O

Pd2(dba)3 Et3N

I

O

O O

DMF 140 °C

EDCI DMAP CH2Cl2 (80%)

O

7

O O

(67%)

20

22

O 21

O

MeO

MeO

MeO A

1) Ph3CBF4 CH2Cl2

O

NHMeBn

2) (PhSeO)2O O NaHCO3 PhCl

HO

NMeBn

THF

O

23

NMeBn C

O

(60%)

O E

O 24

O

O 25

MeO

14

MeO OMe LiAlH4

NMeBn

O 14

THF

NMeBn

O

MeO

7

(77% for 2 steps)

8

NMe2 decaline 215 °C

27

+

NMe2 O (49%)

MeO

OH 26

NMeBn

O

(32%) 28

Scheme 5 Synthesis of A–C–E ring of codeine by Guillou

other variants of Claisen rearrangement [31] (Kazmaier [32], Ireland [33], and Johnson [34]) gave no rearranged product. The dimethylamide function in 27 was reduced by the action of Ti(Oi-Pr)4 and phenylsilane [35] to give aldehyde 29, which was then treated with p-TsOH in toluene at room temperature (Scheme 6). Under these reaction conditions, the intramolecular FriedelCrafts-type cyclization occurred, and the subsequent dehydration of a benzylic alcohol also took place to afford phenanthrofuran skeleton (A–B–C–E ring) 30. The formation of the B-ring by way of intramolecular FriedelCrafts acylation was first reported by Ginsburg in his synthesis of dihydrothebainone in 1954 [36]. This transformation was also employed by White in the synthesis of (þ)-morphine [37], and by Multzer in the synthesis of dihydrocodeinone [38–40]. The FriedelCrafts-type cyclization with an aldehyde function was used by Evans [41] in the formal synthesis of morphine and by Hudlicky in the

8

N. Chida

MeO

MeO

MeO NMeBn

O

Ti(Oi-Pr)4 PhSiH3

H O

O

O

TsOH

NMeBn toluene

NMe2

OH NMeBn

O 27

29 MeO

MeO OH

O H NMeBn

– H2O

O

(40% from 27)

H

NMeBn

30

Scheme 6 Construction of the B-ring by the FriedelCrafts-type cyclization

synthesis of 10-hydroxy-14-epi-dihydrocodeinone [25]. The FriedelCrafts-type cyclization/dehydration with an aldehyde was reported by Ogasawara [42] in the synthesis of (–)-morphine, and later employed by Chida in the synthesis of (–)dihydroisocodeine (see Sect. 2.2.4). The N-benzyl group in 30 was removed by the action of 1-chloroethyl chloroformate [43], and the resulting secondary amine was protected as a sulfonamide to afford 31 (Scheme 7). Allylic oxidation of 31 with SeO2 and t-BuOOH [44] gave allylic alcohol 32 in regio- and stereoselective manners. The similar allylic oxidation of phenanthrofuran compound has been reported by Trost (see Sect. 2.2.1). The stereochemistry of the allylic alcohol in 32 was inverted by oxidationreduction procedure to give 33 in 28% yield from 31. Final ring-closure of D-ring was carried out using the hydroamination of a nitrogen radical (or anion) with a styrene moiety, first reported by Parker in the synthesis of dihydroisocodeine (see Sect. 2.2.2). Thus, treatment of sulfonamide 32 with Li in liquid ammonia in the presence of t-BuOH removed the Ts group on the nitrogen and induced the D-ring cyclization to give racemic codeine in 51% yield. The similar cyclization reactions of the corresponding amine which also provided codeine by the action of LDA and visible light (Trost, see Sect. 2.2.1) and Hg(OAc)2/LiAlH4 (Hudlicky, see Sect. 2.2.3) have been reported. 2.1.3

Stork Group

The Stork group reported the synthesis of racemic codeine and thebaine utilizing the intramolecular Diels–Alder reaction as the key transformation in 2009 [45]. The key features of their synthesis are (1) a direct construction of a phenanthrofuran skeleton (A–B–C–E tetracyclic system) by the intramolecular Diels–Alder reaction of a diene tethered to the benzofuran ring and (2) a formation of the D-ring by the intramolecular SN2 reaction of an amino-mesylate.


1) ClCO2CH(Cl)CH3 (ClCH2)2 then MeOH

O H

NMeBn

MeO

O

2) TsCl, Et3N CH2Cl2 (83%)

H

NMeTs

MeO

MeO 1) DMP CH2Cl2 NMeTs 2) NaBH 4 H MeOH

6

HO 32

SeO2 t-BuOOH dioxane

31

30

O

9

(28% from 31)

MeO Li/NH3 t-BuOH

O H HO

NMeTs

(51%) 33

O

THF

H

NMe

HO codeine

Scheme 7 Synthesis of racemic codeine by Guillou

Reaction of the known acetal 34 readily available from iodoisovanillin with methyl propiolate gave oxy-Michael product 35, whose intramolecular Heck reaction cleanly provided benzofuran 36 (Scheme 8). One-carbon homologation of 36 gave 37. Addition of an E-vinylzirconium species generated by hydrozirconation [46] of 3-buten-1-yne to aldehyde 37, followed by O-silylation, afforded diene 38. When a decaline solution of diene-benzofuran 38 was heated at 230 C in the presence of Et3N in a sealed flask, the desired Diels–Alder reaction took place to afford phenanthrofuran 40 in 89% yield as an epimeric mixture at C-9. The cycloaddition reaction proceeded via the endo-benzofuran transition structure 39, thus providing the adducts in a highly stereoselective manner. It is important to note that the intramolecular Diels–Alder reaction successfully constructed the highly functionalized phenanthrofuran (A–B–C–E ring of morphine), possessing the correct stereochemistry including the quaternary carbon, stereoselectively in a onestep reaction. The intramolecular Diels–Alder reaction of amidofurans tethered onto a benzofuran ring which delivered the simple model compound of an A–C–E ring in morphine skeleton was reported by Padwa [47]. The intermolecular version of Diels–Alder cycloaddition for the construction of A–B–C ring of morphine skeleton was employed by Gates [5, 6] and Tius [48] as the key reactions in their syntheses of morphine. The ester group in 40 was converted into a vinyl ether to give 41, the O-TES protecting group of which was removed and the resulting secondary alcohol was oxidized to afford ketone 42 (Scheme 9). As a tetracyclic structure of 42 allowed the approach of the reagent from the less hindered b-face, the reduction of the ketone carbonyl in 42 with structurally bulky L-selectride (lithium tri-secbutylborohydride) gave the single a-alcohol, which was isolated as its O-methanesulfonate ester 43. Hydrolysis of the vinyl ether function in 43, followed by reductive amination with methylamine in the presence of NaBH4, afforded ethanamine 44.

10

N. Chida

MeO

MeO CO2Me O

HO I

O

Et3N THF (89%)

O

I

O

1) Pd(OAc)2, Ph3P NaOAc MeO Bu4NCl O DMF,125 °C O 2) aq HCl

CO2Me 35

34

CHO

THF

CO2Me

(82%) 36

1) 1) Ph3PCH2OCH3Cl KHMDS, THF

ZrCp2(H)Cl AgOTf CH2Cl2

MeO

2) aq HCl/THF (95%)

CHO O

MeO Et3N 230 °C

OTES

O

2) TESCl imidazole (92%)

CO2Me

37

MeO

CO2Me 38

MeO A

O

decaline

(89%) OTES

H

CO2Me

B

O

O E

9

OTES OMe

α-OTES : β-OTES =1:4

C 40

39

Scheme 8 Construction of a tetracycle of codeine by the intramolecular cycloaddition

MeO

MeO O

O H

OTES OMe

40

1) LiEt3BH THF 2) DMP CH2Cl2 3) Ph3PCH2OCH3Cl KHMDS CH2Cl2 /THF (86%)

MeO

OTES

H 42

OMe

1) L-Selectride THF

O

2) MsCl, Et3N CH2Cl2 (60%)

2) DMP CH2Cl2

H

(88%)

41

MeO

MeO

O

1) TBAF THF

OMe

O

OMe

O H 43

Scheme 9 Preparation of a tetracyclic amine

1) aq HCl THF

NHMe

O

OMs 2) MeNH2•HCl Et3N Ti(Oi-Pr)4 MeOH 3) NaBH4 (87%) 44

OMs H


MeO

MeO NHMe

O

OMs H

K2CO3 benzene 75 °C

1) ClCO2Me NaHCO3 CHCl3

MeO

H

O H

NCO2Me

NHMe

O

NMe

H 46

(30%)

2) SeO2 t-BuOOH CH2Cl2/H2O (60%)

1) PCC MS4A CH2Cl2

HO

+

O

45 (53%)

44

6

11

2) LiAlH4 Et2O (71%)

19

MeO

O H

NMe

HO codeine

Scheme 10 Synthesis of codeine by Stork

Heating of the benzene solution of 44 and K2CO3 induced the intramolecular SN2 reaction to afford 6-deoxycodeine 45 in 53% yield (Scheme 10). The major side product in this reaction was, as anticipated, diene 46 (30% yield), which was formed by elimination reaction. It was reported that the solvent effect is significant in the SN2 reaction: when this reaction was carried out in acetonitrile, diene 46 was obtained as the major product. Treatment of 45 with methyl chloroformate provided a methyl carbamate, whose allylic oxidation with SeO2 in the presence of t-BuOOH introduced the hydroxy group at C-6 to give 19. PCC oxidation of 19, followed by LiAlH4 reduction, provided racemic codeine. The more straightforward approach to codeine methyl ether (52) was also reported by the Stork group (Scheme 11). Hydrozirconation of 4-methoxy3-butene-1-yne and in situ addition of the resulting organozirconate to aldehyde 37 afforded 47 after O-silylation. The intramolecular Diels–Alder reaction of 47 proceeded via the endo-benzofuran transition structure as observed in the reaction of 38, to provide 48 in 69% yield as a mixture of epimeric alcohols at C-9. By this cycloaddition, the requisite four contiguous chiral centers including a quaternary carbon and an alcohol function at C-6 in codeine were successfully constructed stereoselectively. The same reaction sequence as employed for the conversion of 40 to 44 was applied to 48 to give amine 51 via 49 and 50. Similar intramolecular SN2 reaction of 51 as used for the ring closure of 44 afforded codeine methyl ether (52) in 73% yield. The presence of 6-O-methoxy group suppressed the undesired elimination reaction and gave the cyclized product 52 in good yield. Codeine methyl ether (52) could be converted into thebaine by the known onestep procedure (MnO2 oxidation) reported by Rapoport (Scheme 12) [49]. On the

12

N. Chida OMe 1) ZrCp2(H)Cl AgOTf CH2Cl2

MeO CHO O 37

MeO

OMe

O

2) TESCl imidazole (95%)

CO2Me

Et3N 240 °C

OTES

decaline (69%)

CO2Me

47

MeO

MeO 1) LiEt3BH, THF 2) DMP, CH2Cl2

O

O

O

9

6

H

OTES OMe

3) Ph3PCH2OCH3Cl KHMDS CH2Cl2/THF MeO 4) TBAF, THF 5) DMP, CH2Cl2

MeO 48 α-OTES : β-OTES =1:4

H

OMe

1) L-Selectride THF

O

2) MsCl, Et3N CH2Cl2 (34%)

49

(51%)

MeO

MeO OMe

O

OMs H MeO 50

MeO

1) aq HCl THF

K2CO3

NHMe

O 2) MeNH2•HCl Et3N 6 Ti(Oi-Pr)4 MeO MeOH 3) NaBH4 51 (80%)

benzene 75 °C (73%)

OMs H

O 6

MeO 52

Scheme 11 Synthesis of codeine methyl ether by the Stork group MeO

MeO Ref. 49

O H

NMe

MeO

O

MnO2, THF (80%)

NMe MeO

52

thebaine

1) ClCO2Me 2) SeO2 t-BuOOH NaHCO3 CH2Cl2/H2O CHCl3

BBr3 (low yield)

(65%) MeO

MeO LiAlH4

O H O

NCO2Me

Et2O

O H

(82%) HO

53

codeine

Scheme 12 Conversion of codeine methyl ether to thebaine and codeine

NMe

H

NMe


13

other hand, the attempted direct conversion of 52 into codeine by the cleavage of methyl ether with BBr3 resulted in the low yield formation of codeine. To improve this transformation, compound 52 was first converted into methyl carbamate. Oxidation of the carbamate with SeO2 and t-BuOOH provided ketone 53. Finally, reduction of 53 with LiAlH4 gave racemic codeine.

2.1.4

Magnus Group

In 2009, the Magnus group disclosed the synthesis of racemic codeine [50]. In their synthesis (1) an intramolecular phenol alkylation generating an A–C bicyclic ring system with a quaternary carbon, (2) a stereo-controlled, one-pot formation of a B-ring by way of the combination of Henry reaction and Michael addition of nitromethane with a cyclohexadienone-aldehyde, and (3) an introduction of the requisite functionalities in the C-ring by the epoxide-mediated selenylation followed by oxidation, were employed as the key transformations. The SuzukiMiyaura coupling of arylbromide 54 with the boronic acid tris anhydride 55 gave biphenyl 56, which was then converted into bromo-acetal 57 (Scheme 13). Treatment of 57 with CsF in DMF at 130 C induced the intramolecular phenol alkylation to afford 58 in 96% yield. When cyclohexadienone-aldehyde 58 was reacted with nitromethane in acetic acid in the presence of NH4OAc, the Henry reaction/dehydration [51] and the Michael addition of a carbanion of the nitromethane took place simultaneously to generate an A–B–C ring system of morphine 59 in 97% yield in a one-pot reaction. The 1,4-reduction of the nitroalkene moiety in 59 with NaCNBH3 in acetic acid afforded 60 stereoselectively; the stereochemistry at C-9 was controlled by the axial protonation of the intermediary nitronate anion, giving the thermodynamically stable product. The intriguing approach using nitromethane as a one-carbon unit for the construction of B-ring and the masked amino functionality effectively generated the tricyclic system. Further reduction of 60 with LiAlH4 provided amine-alcohol 61. Treatment of 61 with NaCNBH3 in acidic media caused the intramolecular reductive amination of an amine-aldehyde formed by the hydrolysis of an acetal group, as well as the formation of dihydrofuran ring presumably via an allylic cation intermediate generated by dehydration of an allylic alcohol moiety, to afford the pentacyclic morphine skeleton 62. It is important to note that the pentacyclic core structure of morphine was stereoselectively constructed in relatively short reaction steps and with good overall yield (seven steps and 38% yield from 54). Amine 62 was transformed into ethylcarbamate 63, which is the known synthetic intermediate of morphine reported by Taber in 2002 [52]. The pentacyclic core 63 was converted into codeine by a different approach from that of Taber (Scheme 14). Treatment of 63 with dibromohydantoin generated bromohydrin 64 stereoselectively. In this reaction, the reagent approaches from the less hindered b-face and the resulting b-cyclic bromonium ion was opened with water in trans-diaxial manner. Under the reaction conditions, the C-1 position of aromatic ring was concomitantly brominated. Exposure of 64 to KOH/dioxane

14

N. Chida OTBS MeO Pd2(dba)3 PCy3, BHT K2CO3

MeO

+ HO

O B

CHO Br 54

B O

O B

EtO

CH2Cl2

OTBS

OTBS

56

MeO OEt Br2 i-Pr2NEt

MeO

O

CHO

O

CsF

(99%) OTBS 57 MeO

AcOH reflux (97%)

O 58 MeO

MeO CHO

EtO

O CHNO2

MeNO2 NH4OAc

CHO

EtO DMF 130 °C (96%)

Br

O

CHO

aq dioxane 80 °C (99%)

55

TBSO

HO

O

NaCNBH3

EtO

H

NO2

AcOH THF

9

EtO

H

NO2

(88%) O

THF (72%)

MeO

MeO

MeO LiAlH4

O 60

O 59

1M HCl NaCNBH3

O EtO

H OH

NH2

ClCO2Et Et3N

O

dioxane H

(66%) 62

NH

O

CH2Cl2

H

(90%)

NCO2Et

63

61

Scheme 13 Synthesis of a tetracyclic core of morphine by the intramolecular SN2 reaction

afforded a-epoxide 65. Diaxial opening of the epoxide ring in 65 with PhSNa gave 66, which was oxidized by the action of H2O2 to afford diastereomeric sulfoxides 67. Without purification, sulfoxides 67 were heated in toluene in the presence of NaHCO3 to give elimination product 68 along with a saturated ketone 69 in 76% and 17% isolated yields, respectively. On the other hand, the same transformation using PhSeNa, instead of PhSNa, provided 68 as the sole product in 72% from 65. Finally, treatment of 68 with LiAlH4 reduced the ethylcarbamate function and removed the C-1 bromine atom to afford codeine.


15

O MeO

NBr

BrN

O aq dioxane (97%)

O H

NCO2Et

2

MeO

1

KOH O dioxane NCO2Et H reflux (91% for 2 steps)

O

HO

63

Br

H O

Br 64

MeO

Br

MeO

O H

NCO2Et

HO

H

CF3

65 PhSeNa EtOH (93%) Br

PhSNa EtOH (99%)

MeO

NCO2Et

HO

H

67

SePh

66

70 NaIO4 THF

Br

MeO

+

O H

NCO2Et

HO SPh

NaHCO3 toluene reflux MeO

NCO2Et

O

OH F3C

SPh

Br

O

H2O2

O

MeO

Br

NaHCO3

O

NCO2Et

O

MeO

Br

H

NCO2Et

(77% from 70)

HO 69 (17% from 66)

toluene reflux

Br

O H

NCO2Et

HO

68 (76% from 66)

SePh O

LiAlH4 THF (87%)

71

MeO

O H

NMe

HO codeine

Scheme 14 Synthesis of codeine by Magnus

2.2

Chiral Synthesis

The synthesis of biologically active natural products in optically active form is highly important since, in many cases, enantiomers of natural products show no or different biological activities. In this section, chiral syntheses of morphine-related

16

N. Chida

compounds reported since 2005 are discussed. Interesting and independent approaches to the target compounds in optically active forms, a Pd-catalyzed asymmetric alkylation by Trost, an asymmetric reduction of ketone carbonyl by Parker, an enzymatic oxidation of benzene derivative by Hudlicky, and a chiral pool approach by Chida have been reported. Earlier works of enantioselective syntheses of morphine-related compounds have been reviewed by Taber [14].

2.2.1

Trost Group

In 2002, Trost and Tang reported the chiral total synthesis of (–)-codeine in short reaction steps using a palladium-catalyzed asymmetric allylic alkylation (AAA) [53] as the key transformation [54]. In 2005, a detailed full account of their synthesis was published [55]. The key features of their synthesis are (1) a preparation of an aryl ether with high optical purity by the Pd-catalyzed AAA reaction, (2) the intramolecular Heck reaction to generate the A–C–E benzofuran skeleton, (3) the second intramolecular Heck reaction of Z-vinyl bromide providing the phenanthrofuran core, and (4) the intramolecular hydroamination for the construction of D-ring by the action of LDA and visible light. The Pd-catalyzed AAA reaction of bromophenol 54 and allylic carbonate 72 in the presence of chiral bis-phosphine ligand 73 gave aryl ether 74 in 72% yield with 88% ee (Scheme 15). After protection of the aldehyde function in 74 as a dimethyl CHO [(η3-C3H5)PdCl]2 Et3N, 73

COOMe

OHC TrocO Br

OMe

+

CH2Cl2

OH 54

72

Ph

Br COOMe

(72%)

Ph

O

O NH

MeO

O 74 88% ee

HN

PPh2 Ph2P 73 1) TsOH CH(OMe)3 MeOH 2) DIBALH toluene

CHO Br

CN

3) acetonecyanohydrin O MeO Ph3P, DIAD Et2O 4) TsOH aq THF 75 96% ee after (65%) recrystallization

Pd(OAc)2 dppp Ag2CO3

MeO CHO CN

O

toluene 107 °C (91%)

Scheme 15 Preparation of optically active A–C–E ring of codeine by Trost

76


17

acetal, the ester function was reduced to a primary alcohol. The b,g-unsaturated nitrile was synthesized by the Mitsunobu reaction. Acid hydrolysis of the dimethyl acetal followed by recrystallization provided 75 with 96% ee. Intramolecular Heck reaction produced the A–C–E ring system 76 possessing a quaternary carbon with correct stereochemistry in 91% yield (see Sect. 2.1.1). CoreyFucus olefination [56] of 76 followed by regioselective debromination [57] afforded Z-vinyl bromide 77 (Scheme 16). The intramolecular Heck vinylation successfully constructed the B-ring to provide phenanthrofuran skeleton 78 in 65% yield. The success in the formation of the tetracyclic core fully revealed the effectiveness of the Heck reaction for the stereoselective preparation of the morphine skeleton. Allylic oxidation of 78 with SeO2 gave a mixture of an allylic alcohol and an a,b-unsaturated ketone. This mixture was treated with DMP to give ketone 79. The 1,2-reduction of 79 with DIBALH in THF-Et2O gave the desired a-alcohol 80. Reaction of 80 with DIBALH in CH2Cl2-Et2O reduced the nitrile function to give an imine, which was then treated with NH4Br and excess methylamine in methanol to give a secondary imine. Without isolation, NaBH4 reduction MeO

MeO CHO CN

O

1) CBr4 PPh3 CH2Cl2

Br

O

CN

2) nBu3SnH Pd(PPh3)4 toluene

76

MeO Pd(OAc)2 dppp Ag2CO3

O

CN

toluene

H

(65%) 77

(80%)

78 MeO

MeO DIBALH

SeO2

O

1,4-dioxane 75°C then DMP

CN

THF/Et2O

H

O

CN H

(99%) HO

O 79

(58%)

80

1) DIBALH CH2Cl2 /Et2O

1) DIBALH CH2Cl2/Et2O 2) NH4Br MeNH2 MeOH then NaBH4 (100%)

2) NH4Br MeNH2 MeOH then NaBH4 (89%) MeO

MeO LDA Tungsten bulb

O H

NMe

HO (–)-codeine

Scheme 16 Synthesis of ()-codeine

O NHMe

THF (57%)

H HO 81

18

N. Chida

of the mixture afforded amine 81, quantitatively. Alternatively, reduction of 79 with DIBALH in CH2Cl2-Et2O, followed by the treatment with NH4Br and methylamine and subsequent reduction with NaBH4 also afforded 81 in one-pot reaction. Subjecting the solution of 81 and LDA in THF to irradiation with a 150-W tungsten light bulb at room temperature led to the intramolecular hydroamination to afford (–)-codeine in 57% yield. Since the similar reaction without the irradiation gave no cyclized product, the Trost group suggested that the visible light might have promoted the single electron transfer to facilitate the cyclization (see Sect. 2.1.2). 2.2.2

Parker Group

In 1992, Parker and Fokas reported the short step (11 steps) synthesis of racemic dihydroisocodeine, which completes a formal synthesis of codeine and morphine [58]. The key feature of their synthesis are (1) a construction of an aryl ether moiety (connection of the A and C rings) by Mitsunobu reaction, (2) a tandem radical cyclization of aryl bromide possessing the C-ring precursor to generate the A–B–C–E ring of morphine, and (3) a hydroamination for the construction of the D-ring by the reaction of a tosylamide with Li/NH3. In 2006, they reported the chiral version of the synthesis of dihydrocodeinone [59]. m-Methoxyphenethylamine 82 was employed as the starting material (Scheme 17). Birch reduction of 82 gave dienol ether 83. N-Tosylation followed by hydrolysis and subsequent N-methylation afforded cyclohexenone 84. To obtain chiral cyclohexenol 87, whose racemic compound had been used as the key intermediate in their racemic synthesis of dihydroisocodeine, cyclohexenone 84 was subjected to various asymmetric reduction conditions. Chiral reduction of 84 with Terashima’s reagent (LiAlH4/(–)-N-methylephedrine/2-ethylaminopyridine) [60] and the CoreyBakshiShibata (CBS) reagent [61] afforded the desired (R)-cyclohexenol 87; however, the optical purity of the product was found to be NH2

NH2 1) TsCl, Et3N THF then 1 M aq HCl

Li/NH3 t-BuOH -68 °C

MeO

(97%)

82

2) MeI, K2CO3 O acetone (78%)

MeO 83

H Ph Ph O N B Me catechol borane toluene/THF -78 °C to rt (84%)

O 85

(91%)

84

asymmetric reduction NMeTs

NMeTs Br HO

Na-Hg THF/MeOH 86

Br2 Et3N

CH2Cl2

low ee

NMeTs Br

NMeTs

(90%)

HO 87

82% ee

Scheme 17 Preparation of an optically active cyclohexenol by asymmetric reduction


19

unsatisfactory (5% ee with Terashima’s reagent and 35–40% ee with CBS reagent). For improvement of the optical purity of 87, compound 84 was then converted into 2-bromocyclohexenone derivative 85, since it has been reported that the CBS reduction of 2-bromocyclohexenone gave the product with high (91%) ee [62]. Indeed, reduction of 85 with the CBS reagent [(S)-oxazaborolidine and catechol borane] afforded bromo-cyclohexenol 86 in 84% yield with 82% ee. The similar CBS asymmetric reduction was also employed in the synthesis of optically active dihydroisocodeine by Overman [21]. Treatment of 86 with sodium amalgam provided 87 in 90% yield. With the chiral cyclohexenol 87 in hand, the synthesis of optically active morphine was carried out based on the similar reaction sequence as employed for their racemic synthesis (Scheme 18). Treatment of 87 with m-CPBA gave NMeTs

NMeTs NMeTs CH2Cl2 (86%)

HO

1) Ti(Oi-Pr)4 benzene

O

m-CPBA

2) TBDMSOTf TBDMSO i-Pr2NEt CH2Cl2 (61%)

HO

87

HO

88

1) MeO

89

SPh

HO

SPh

NMeTs

Br 90 PBu3, DEAD, THF

Br MeO

Bu3SnH AIBN

O

benzene 130 °C in a sealed tube (30%)

2) 10% HF/CH3CN (65%)

HO 91

MeO

MeO

MeO NMeTs

NMeTs O

O

O SPh

NMeTs

SPh

HO

H HO

HO

92 MeO Li/NH3 t-BuOH

Swern oxidation

O

THF –10 °C (80%)

MeO

H

NMe

HO (–)-dihydroisocodeine (93)

O

(80%) H

NMe

O (–)-dihydrocodeinone (94) 75% ee

Scheme 18 Synthesis of ()-dihydrocodeinone by Parker

20

N. Chida

b-epoxide 88. Regioselective isomerization of 88 with Ti(Oi-Pr)4 followed by O-silylation afforded 89. Mitsunobu coupling of 89 with phenol 90 and subsequent deprotection of the silyl group cleanly gave aryl ether 91. When 91 was reacted with Bu3SnH and AIBN in benzene at 130 C, compound 91 underwent the projected tandem radical cyclization/elimination sequence to give phenanthrofuran compound 92 (A–B–C–E ring system of morphine) in 30% yield in one-pot reaction. Treatment of 92 with Li/NH3 removed the N-Ts group to generate the nitrogen radical (or anion), which spontaneously added to the b-carbon of the styrene moiety, affording dihydroisocodeine (93) directly in 80% yield (see Sects. 2.1.2 and 2.2.1). Swern oxidation of 93 provided dihydrocodeinone (94) in 80% yield. Comparison of [a]D values of synthetic 94 with that of the authentic sample revealed that dihydrocodeinone obtained in this synthesis had 75% ee. Since dihydroisocodeinone had been successfully converted into morphine [63, 64], synthesis of (–)-morphine in the formal sense has been achieved.

2.2.3

Hudlicky Group

In 2007, the Hudlicky group reported the total synthesis of (þ)-codeine (an enantiomer of the natural product) starting from a chiral cyclohexadiene-diol derivative obtained by the enzymatic oxidation of b-bromoethylbenzene [65]. Later, total synthesis of (–)-codeine starting from the same chiral cyclohexadienediol was reported in 2009 [66]. The key features of their synthesis involve (1) an enantio-divergent synthesis of (þ)- and ()-codeine starting from the same cyclohexene derivative, (2) the intramolecular Heck reaction to generate the A–C–E tricyclic system including the stereoselective generation of a quaternary carbon center, (3) the second Heck reaction of a vinyl bromide for the construction of a phenanthrofuran core, and (4) the mercury(II)-mediated intramolecular aminomercuration/demercuration for the final D-ring closure. Diimide reduction of 96, obtained by the enzymatic oxidation of 95 (96% ee) [67], followed by O-acetylation afforded cyclohexene-diol 97 in 47% yield (Scheme 19). Compound 97 was converted into carbamate 98 by SN2 reaction with methylamine and subsequent urethane formation. Regioselective protection of the diol in 98 with TBSCl afforded 99, which was coupled with bromoisovanillin 54 under the conditions of Mitsunobu to provide 100 (see Sect. 2.2.2). The intramolecular Heck reaction of 100 constructed the benzofuran ring including a quaternary center to afford 101 in 82% yield. Wittig reaction of 101 with Ph3P¼CHBr generated vinyl bromide 102 in 49% yield as a mixture of E- and Z-isomers (E:Z ¼ ca. 1:2). The second Heck reaction of 102 (E/Z isomers) utilizing the vinyl bromide function, which had been employed in the synthesis of codeine by Trost (see Sect. 2.2.1), afforded phenanthrofuran skeleton 103 in 44% yield (Scheme 20). Since only the Z-isomer of 102 can participate in the cyclization reaction, this Heck cyclization resulted in the rather modest yield. After deprotection of the O-TBS group in 103, the stereochemistry at C-6 hydroxy group was inverted by


enzymatic dihydroxylation

Br OH

1) KO2CN=NCO2K AcOH/MeOH

Br OAc

E. coli JM109 phosphate buffer

OH

2) Ac2O, Et3N DMAP, CH2Cl2

OAc

Br

95

21

96 96% ee

97

(47%)

MeNH2 K2CO3 THF then Boc2O DMAP CH2Cl2 (50%)

CHO NMeBoc TBSCl OH imidazole

NMeBoc OH

Bu3P, DIAD THF

Br NMeBoc

14 8

CH2Cl2

OH

MeO

MeO

OTBS 99

98

HO

CHO Br 54

100

MeO Ph3PCH2Br2 t-BuOK

CHO

O

toluene

NMeBoc

(82%)

TBSO

(55%)

MeO Pd(OAc)2 Ag2CO3 dppf

O

THF

TBSO

Br

O NMeBoc

(49%) TBSO

101

102 (E : Z = 1 : 2)

Scheme 19 Preparation of A–C–E ring of (+)-codeine by Hudlicky MeO

O

Br

Pd(OAc)2 Ag2CO3 dppp

MeO

O

toluene NMeBoc (44%)

6

TBSO

TBSO

102 (E : Z = 1 : 2)

1) TBAF MeO THF 2) IBX DMF O 3) NaBH4 NMeBoc MeOH H (72%) HO

TFA

NMeBoc

104

103

MeO

CH2Cl2

H

6

MeO Hg(OAc)2 Et3N

O H

(88%)

NHMe

HO

(18%) ent- 81

O

THF then LiAlH4

H

NMe

HO (+)-codeine

Scheme 20 Synthesis of (+)-codeine

oxidationreduction procedure to give 104. Removal of the N-Boc protecting group in 104 generated tetracyclic amine ent-81, which is an enantiomer of the known synthetic intermediate reported by Trost (see Sect. 2.2.1). Following Trost’s protocol, ent-81 was treated with LDA with irradiation of visible light. However, the reproducibility of the hydroamination was found to be rather poor. To solve the

22

N. Chida

problem, the Hudlicky group adopted the intramolecular aminomercuration/demercuration procedure. Thus, treatment of ent-81 with Hg(OAc)2 followed by reduction with LiAlH4 successfully provided (þ)-codeine, an enantiomer of the natural product, in 18% yield. Starting from diol 98, total synthesis of (–)-codeine was also accomplished (Scheme 21). Mitsunobu reaction of 98 with p-nitrobenzoic acid gave an inverted ester 105 regioselectively. It is interesting to note that the silylation of 98 gave 8-O-TBS derivative 99 exclusively (Scheme 17), whereas the Mitsunobu conditions activated another (C-14) hydroxy group. The Hudlicky group suggested that the observed reverse regioselectively might be explained by the anchimeric assistance of the carbamate function. The alcohol in 105 was tosylated to give 106, which was then treated with base to give a-epoxide 107. Reaction of 107 with potassium salt of bromoisovanillin afforded aryl ether ent-100 regioselectively in 46% yield. The intramolecular Heck reaction produced tricycle ent-101 in 91% yield. The same reaction sequence as employed for the conversion of 101 to 103 was applied to ent-101 to give tetracycle ent-103 in 18% yield (two steps) (Scheme 22). Alternatively, Wittig reaction of ent-101 with Ph3P¼CHI afforded vinyl iodide 108 in better yield (72%, E:Z ¼ 1:2). The Heck reaction of 108, however, resulted in the formation of ent-103 with lower yield (31%) than that of ent-102. The attempted new procedure via vinyl iodide 108 slightly improved the overall yield of ent-103 from ent-101 (22% for two steps). Inversion of the C-6 hydroxy group and deprotection of N-Boc group gave methylamine 81. Aminomercuration/ demercuration of 81 provided (–)-codeine in 24% yield. 2.2.4

Chida Group

In 2008, the Chida group reported the synthesis of (–)-dihydroisocodeine starting from D-glucal [68]. This report, based on the “chiral pool” approach, would be the NMeBoc

OH 14 8

OH 98

NMeBoc

NMeBoc

p-nitrobenzoic acid PPh3, DIAD

TsCl Et3N OBz(p-NO2) DMAP

THF (71%)

CH2Cl2

OH

(73%)

105

OBz(p-NO2)

NaOMe

OTs

MeOH (76%)

106

MeO 1) NMeBoc KO

CHO

Br 18-crown-6 DME/DMF O 107

MeO

CHO

2) TBSCl imidazole CH2Cl2 (46%)

Br MeO

O

Pd(OAc)2 NMeBoc Ag2CO3 dppf toluene (91%)

TBSO ent-100

Scheme 21 Preparation of A–C–E ring of ()-codeine

CHO

O

NMeBoc TBSO ent-101

Recent Advances in the Synthesis of Morphine and Related Alkaloids MeO Ph3PCH2Br2 t-BuOK THF (40%)

MeO

Br Pd(OAc)2 Ag2CO3 dppp NMeBoc toluene MeO (45%)

O

TBSO

ent-102 (E : Z = 1 : 2)

CHO

O

23

MeO

NMeBoc

O I

H

6

NMeBoc

TBSO

TBSO ent-101

Ph3PCH2I2 NaHMDS HMPA THF (72%)

1) TBAF THF (72%) 2) IBX DMF (46%) 3) NaBH4 MeOH (76%) 4) TFA CH2Cl2 (90%)

ent-103

O

Pd(OAc)2 Ag2CO3 NMeBoc dppp toluene TBSO 108 (31%) (E : Z = 1 : 2) MeO

MeO Hg(OAc)2 Et3N

O H

6

HO 81

NHMe

O

THF then LiAlH4 (24%)

H

NMe

HO (–)-codeine

Scheme 22 Synthesis of ()-codeine

first example of the chemical synthesis of optically pure morphine starting from an enantiomerically pure compound. The key features of their synthesis are (1) a use of carbohydrate as a chiral building block for the C-ring precursor, (2) a construction of A–C ring by way of SuzukiMiyaura coupling in good yield, (3) the cascade Claisen rearrangement creating the vicinal tertiary and quaternary carbon centers in morphine skeleton stereoselectively in a one-pot reaction, and (4) a construction of B-ring by way of the FriedelCrafts-type cyclization/dehydration under mild acidic conditions. Treatment of D-glucal, obtained by methanolysis of commercially available triO-acetyl-D-glucal 109, with p-anisaldehyde dimethylacetal produced a 4,6-O-benzylidene derivative, whose hydroxy group was protected as a TBS ether to give 110 (Scheme 23). Reductive opening of a benzylidene group by DIBALH and subsequent treatment of the product with Ph3P·HBr in methanol provided methyl glycoside, which was transformed into primary iodide 111. Elimination of HI provided enopyranoseide 112. Treatment of 112 with a catalytic amount of Hg (OCOCF3)2 in aqueous acetone induced the catalytic Ferrier’s carbocyclization [69–71] to give cyclohexenone 113, which was cleanly converted into cyclohexenone 114 by the action of MsCl and Et3N. 1,4-Reduction of 114 with L-selectride and subsequent trap of the resulting enolate with Commins reagent gave vinyl triflate 115. SuzukiMiyaura coupling of 115 with 2,3-dimethoxyphenyl boronic

24

N. Chida 1) DIBALH toluene 2) Ph3P•HBr NaBr PMBO MeOH/DME

MeO OAc 1) NaOMe, MeOH 2) p-anisaldehyde PPTS, DMF O

AcO

O O

3) TBSCl, imidazole DMF (45%)

AcO 109

O

3) I2 imidazole Ph3P, THF

TBSO 110

I O OMe

TBSO 111

(69%) O

t-BuOK

PMBO

THF

TBSO

(87%)

Hg(OCOCF3)2 (20 mol%)

PMBO

acetone/ OMe acetate buffer

TBSO

O

112

MsCl, Et3N DMAP OH 113

CH2Cl2 (91% for 2 steps)

MeO O

OTf

PMBO

L-Selectride THF, –78 °C

TBSO

then 114

PMBO TBSO NTf2

N

115

MeO PMBO

Pd(OAc)2, Ph3P aq Na2CO3/dioxane (quant)

TBSO 116

MeO

MeO DDQ

MeO B(OH)2

(89%)

aq CH2Cl2 (83%)

MeO

TBAF

MeO HO

THF

MeO HO

(quant) HO

TBSO 117

118

Scheme 23 Preparation of an enantiomerically pure A–C ring from D-glucal

acid afforded coupling product 116, quantitatively. Deprotection of O-PMB group by the action of DDQ afforded cyclohexenol 117. Further deprotection of 117 with TBAF afforded diol 118. When the mixture of 118 with 2-nitrophenol in triethyl orthoacetate was heated at 140 C in a sealed tube, the cascade Claisen rearrangement took place to afford the doubly rearranged product 119 in 36% yield (Scheme 24). By this cascade sigmatropic rearrangement, the vicinal tertiary and quaternary carbon centers in codeine (C-14 and C-13) were stereoselectively constructed via the sequential chirality transfer. On the other hand, Claisen rearrangement of 117 in the presence of propionic acid afforded the rearranged product 120 in 87% yield. Deprotection of the TBS group afforded 121. The second Claisen rearrangement of 121 in the presence of 2-nitrophenol provided 119 in 48% overall yield (56% overall yield based on the recovered starting material) from 117. They reported that attempted


HO

MeO

MeO

MeO HO

2-nitrophenol

118

CH3C(OEt)3 140 °C in a sealed tube

MeO

25

CO2Et

MeO 13

EtO

O

(36%)

MeO

MeO HO

EtCO2H

TBSO 117

CH3C(OEt)3 140 °C in a sealed tube (87%)

CO2Et CO2Et

119

(57%) (121: 14% recovery) MeO

14

2-nitrophenol CH3C(OEt)3 140 °C in a sealed tube

MeO

MeO

CO2Et TBAF

MeO

CO2Et

THF TBSO

120

(97%)

TBSO

121

Scheme 24 Cascade Claisen rearrangement installing the vicinal tertiary and quaternary carbons

Eschenmoser Claisen rearrangement and Ireland Claisen rearrangement of 118 produced no doubly rearranged product (see Sect. 2.1.2). Treatment of 119 with m-CPBA induced the dealkylating etherfication to generate dibenzofuran skeleton, and the subsequent protection of a hydroxyl group as a TBS ether provided 122 (Scheme 25). The similar epoxide-mediated dealkylating etherfication has been employed in the synthesis of morphine by Multzer [39]. The ester functions in 122 were reduced with DIBALH to give dialdehyde 123. When 123 was treated with montmorillonite K-10 clay [72] in dichloroethane at room temperature, FriedelCrafts-type cyclization took place, and subsequent dehydration of the resulting benzylic alcohol constructed the tetracyclic phenanthrofuran core. Since the TBS protecting group was partially deprotected under these reaction conditions, the reaction mixture was re-silylated to provide 124 in 75% yield from 122. As mentioned in Sect. 2.1.2, the similar FriedelCrafts-type cyclization/ dehydration process for the construction of the B-ring has been reported by Ogasawara in 2002 (ethyleneglycol and CSA in benzene, reflux, 50% yield) and Guillou in 2008 (p-TsOH, toluene, room temperature, 40% yield) in their synthesis of morphine. It should be noted that the current reaction proceeded under very mild reaction conditions with good yield (75%), and another aldehyde function was intact throughout the reaction. Reductive amination of 124 with methylamine smoothly gave 125, whose amino group was tosylated and TBS group was deprotected to give the known synthetic intermediate 92 of morphine reported by Parker (see Sect. 2.2.2). The similar reductive hydroamination of 92 as the Parker’s method provided ()-dihydroisocodeine in 92% yield, completing the formal synthesis of ()-morphine (see Sect. 2.2.2).

26

N. Chida MeO

MeO MeO

119

MeO

1) m -CPBA CH2Cl2 O CO2Et CO2Et 2) TBSCl imidazole TBSO DMF (73%)

1) Montmorillonite K-10 (CH2Cl2)2, rt 2) TBSOTf 2,6-lutidine CH2Cl2

MeO

O

CHO H

(75% from 122)

CO2Et

DIBALH (2 equiv)

CHO

O CO2Et toluene –78 °C TBSO 123

122

MeNH2 MeNH3Cl MS3A

MeO

1) TsCl DMAP pyridine

O

THF then LiBH4

TBSO

CHO

H

(86% form 124)

TBSO 124

NHMe 2) TBAF THF

125

MeO

MeO Li, t-BuOH liq NH3

O H

NMeTs

(92%)

HO 92

O

THF

H

NMe

HO (–)-dihydroisocodeine (93)

Scheme 25 Synthesis of ()-dihydroisocodeine by Chida

3 Closing Remarks The work that culminated in the synthesis of morphine and its related compounds reported from eight research groups during 2004–2009 has been shortly reviewed. It is interesting that each group employed different strategies and key reactions for the construction of a morphine-skeleton. These insightful approaches would be useful and valuable for synthetic organic chemists to plan their own synthetic strategies of morphine-type alkaloids as well as other nitrogen-containing compounds. Morphine, not such a big molecule, but possessing densely functionalized structure with five contiguous chiral centers, has been a good target to test the quality of the strategies and tactics of the synthesis. The synthetic endeavor to such a challenging small but complex molecule based on the imaginative idea sometimes makes a great discovery in organic chemistry. Although the synthetic ways to morphine that have been reported to date may not be truly practical, the more imaginative and creative idea, and most importantly, the enthusiasm for the organic chemistry, surely would give rise to the more efficient synthesis of the alkaloid. There is little doubt that the challenges for organic chemists will continue.


27

References 1. Booth M (1998) Opium: a history. St. Martin’s Press, New York 2. Kapoor LD (1995) Opium poppy: botany, chemistry, and pharmacology. The Haworth, New York 3. Cordell GA (1981) Introduction to alkaloids: a biogenetic approach. Wiley, New York 4. Butora G, Hudlicky T (1998) The story of morphine structure elucidation: one hundred years of deductive reasoning. In: Hudlicky T (ed) Organic synthesis: theory and applications. JAI Press, Stamford, p 1 5. Gates M, Tschudi G (1952) J Am Chem Soc 75:1109 6. Gates M, Tschudi G (1956) J Am Chem Soc 78:1380 7. Maier M (1995) Synthesis of morphine. In: Waldmann H (ed) Organic synthesis highlights II. VCH, Weinheim, p 357 8. Kametani T (1977) The total syntheses of isoquinoline alkaloids. In: ApSimon J (ed) The total synthesis of natural products, vol 3. Wiley, New York, p 1 9. Corey EJ, Cheng XM (1989) The logic of chemical synthesis. Wiley, New York 10. Novak BH, Hudlicky T, Reed JW, Multzer J, Trauner D (2000) Curr Org Chem 4:343 11. Blakemore PR, White JD (2002) Chem Commun 1159 12. Hudlicky T, Butora G, Feamley SP, Gum AG, Stabile MR (1996) A historical perspective of morphine synthesis. In: Atta-ur-Rahman (ed) Natural products chemistry, vol 18. Elsevier, Amsterdam, p 45 13. Zezula J, Hudlicky T (2005) Synlett 388 14. Taber DF, Neubert TD, Schlecht MF (2004) The enantioselective synthesis of morphine. In: Harmata M (ed) Strategies and tactics in organic synthesis, vol 5. Elsevier, London, p 353 15. Mascavage LM, Wilson ML, Dalton DR (2006) Curr Org Synth 3:99 16. Uchida K, Yokoshima S, Kan T, Fukuyama T (2006) Org Lett 8:5311 17. Uchida K, Yokoshima S, Kan T, Fukuyama T (2009) Heterocycles 77:1219 18. Tsuji J (1995) Palladium reagents and catalysis. Wiley, New York 19. Mitsunobu O (1981) Synthesis 1 20. Hughes DL (1992) The Mitsunobu reaction. In: Paquette LE (ed) Organic reactions, vol 42. Wiley, New York, p 335 21. Hong CY, Kado N, Overman LE (1993) J Am Chem Soc 115:11028 22. Cheng CY, Liou JP, Lee MJ (1997) Tetrahedron Lett 38:4571 23. Hsin LW, Chang LT, Chen CW, Hsu CH, Chen HW (2005) Tetrahedron 61:513 24. Hsin, LW, Chang LT, Liou HL (2008) Synlett 2299 25. Omori AT, Finn KL, Leisch H, Carroll RJ, Hudlicky T (2007) Synlett 2859 26. Itoh Y, Hirao T, Saegusa T (1978) J Org Chem 43:1101 27. Barton DHR, Motherwell RSH, Motherwell WB (1981) J Chem Soc Perkin Trans 1:2363 28. Rice KC (1977) J Med Chem 20:164 29. Varin M, Barre E, Iorga B, Guillou C (2008) Chem Eur J 14:6606 30. Wick AE, Steen K, Eschenmoser A (1964) Helv Chim Acta 47:2425 31. Castro AMM (2004) Chem Rev 104:2939 32. Kazmaier U (1994) Angew Chem Int Ed Engl 33:998 33. Ireland RE, Mueller RH (1972) J Am Chem Soc 94:5897 34. Johnson WS, Werthermann L, Barlett WR, Brocksom TJ, Li TT, Faulkner DJ, Petersen MR (1970) J Am Chem Soc 92:741 35. Bower S, Kreutzer KA, Buchwald SL (1996) Angew Chem Int Ed Engl 35:1515 36. Elad D, Ginsburg D (1954) J Chem Soc 3052 37. White DJ, Hrnciar P, Stappenbeck F (1997) J Org Chem 62:5250 38. Mulzer J, Durner G, Trauner D (1996) Angew Chem Int Ed Engl 35:2830 39. Multzer J, Bats JW, List B, Opatz T, Trauner D (1997) Synlett 441 40. Trauner D, Bats JW, Werner A, Mulzer J (1998) J Org Chem 63:5908

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Opioids in Preclinical and Clinical Trials Hiroshi Nagase and Hideaki Fujii

Abstract Since 1952, when Gates determined the stereo structure of morphine, numerous groups have focused on discovering a nonnarcotic opioid drug [1]. Although several natural, semisynthetic, and synthetic opioid ligands (alkaloids and peptides) have been developed in clinical studies, very few were nonnarcotic opioid drugs [2]. One of the most important studies in the opioid field appeared in 1976, when Martin and colleagues [3] established types of opioid receptors (these are now classified into m, d, and k types). Later, Portoghese discovered a highly selective m type opioid receptor antagonist, b-funaltrexamine [4]. This led to the finding that the m type opioid receptor was correlated to drug dependence [5]. Consequently, d, and particularly k, opioid agonists were expected to lead to ideal opioid drugs. Moreover, opioid antagonists were evaluated for the treatment of symptoms related to undesirable opioid system activation. In this chapter, we provide a short survey of opioid ligands in development and describe the discovery of the two most promising drugs, TRK-851 [6] and TRK-820 (nalfurafine hydrochloride) [7]. Keywords Clinical development Nonnarcotic Opioid drug TRK-820 TRK-851

Contents 1

Opioid Ligands in Early Drug Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1 Peripheral Antagonists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 d Opioid Agonists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3 d Opioid Antagonists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4 k Opioid Agonists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

H. Nagase (*) and H. Fujii School of Pharmacy, Kitasato University, 5-9-1, Shirokane, Minato-ku, Tokyo, Japan e-mail: [email protected]

31 31 32 33 34

30

H. Nagase and H. Fujii

Antitussive d Opioid Antagonist TRK-851 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 2.1 Antitussive Effects of Opioid Ligands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 2.2 Mechanism of the Antitussive Effect of NTI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 2.3 Improvement on the Antitussive Activity of NTI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 2.4 Improvement of Metabolic Defect of TRK-850 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 2.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 3 Antipruritic k Opioid Agonist TRK-820 (Nalfurafine Hydrochloride) . . . . . . . . . . . . . . . . . . . . . 48 3.1 Design Rationale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 3.2 Pharmacology and Medicinal Chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 3.3 Clinical Development of TRK-820 in Postoperative Surgery . . . . . . . . . . . . . . . . . . . . . . . . 56 3.4 Clinical Development of TRK-820 as an Antipruritic Agent . . . . . . . . . . . . . . . . . . . . . . . . 57 3.5 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 2

Abbreviations AAW ADME ADR b-FNA BBB BNTX cLog P CNS CTOP DAMGO DPDPE DSLET GPI HPLC i.c. i.d. i.p. i.t. MVD nor-BNI NTI PBS p.o. SAR s.c. S.E.M. TF VAS

Acetic acid writhing Absorption, distribution, metabolism, and excretion Adverse drug reaction b-Funaltrexamine Blood–brain barrier 7-Benzylidenenaltrexone Calculated log P Central nervous system H-D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 [D-Ala2, N-Me-Phe4, Gly-ol5]enkephalin [D-Pen2, D-Pen5]enkephalin Tyr-D-Ser-Gly-Phe-Leu-Thr Guinea pig ileum High-performance liquid chromatography Intracisternal Intradermal Intraperitoneal Intrathecal Mouse vas deferens nor-Binaltorphimine Naltrindole Phosphate-buffered saline Peroral Structure–activity relationship Subcutaneous Standard error of the mean Tail flick Visual analog scale

Opioids in Preclinical and Clinical Trials

31

1 Opioid Ligands in Early Drug Development Representative opioid agonists and antagonists for m, d, and k opioid receptors are listed in Table 1. Recent drug development efforts in the opioid field have mainly focused on improving patient compliance by creating new formulations, including slow release, long acting tablets, and transdermal delivery [8–11]. Others have focused on preventing illegal use by combining an opioid agonist with an opioid antagonist, naloxone, in an oral formulation. Naloxone is immediately metabolized in its first pass through the liver clearance process. Therefore, when the combination drug is used orally, the opioid agonist has a normal analgesic effect. However, when the drug is ground to a powder for intravenous injection, the naloxone works to prevent the effect of the opioid agonist [12]. Recently, relatively small numbers of new opioid compounds have been developed. New chemicals for producing an opioid drug can be categorized into four classes: 1. 2. 3. 4.

Peripheral antagonists d Agonists d Antagonists k Agonists We will provide short reviews of these drug candidates in the following subsections.

1.1

Peripheral Antagonists

Patients that take prescription opioid analgesics often experience intolerable side effects, including nausea, emesis, or constipation. Because opioid analgesics act primarily on the central nervous system (CNS), targeting an antagonist outside the CNS might prevent peripheral side effects without affecting the analgesic effects. Table 1 Representative opioid ligands Type Selective Agonists Antagonists CTOP m DAMGO b-FNA Morphine Fentanyl d DPDPE NTI DSLET NTB SNC-80 BNTX TAN-67 k U-50,488H nor-BNI U-62,066 TRK-820

Nonselective Agonists Antagonists Etorphine Naloxone Levorphanol Naltrexone

Endogenous Endomorphin Endorphin Enkephalin

Dynorphin

32


N

Br OH HO

H N

N HO

O

O

Methylnaltrexone bromide

O OH

O Alvimopan

Fig. 1 Structures of methylnaltrexone bromide and alvimopan

This concept proved to be successful with methylnaltrexone bromide (Fig. 1) [13], which was launched in 2008 by Wyeth (originated at the University of Chicago) for the treatment of constipation caused by opioid analgesics. The addition of a quaternary cationic 17-nitrogen moiety prevented the compound from passing through the blood–brain barrier (BBB), but also reduced the antagonistic activity. Other examples of peripheral antagonists include Alvimopan (Fig. 1) [14] and S-2979951 (in Phase I by Shionogi; chemical structure undetermined). Alvimopan was launched in 2008 by GSK (originated by Adolor) for recovery of normal gastrointestinal function after abdominal surgery and to accelerate upper and lower gastrointestinal recovery after partial large or small bowel resection surgery with primary anastomosis.

1.2

d Opioid Agonists

Currently, there are no d opioid pharmaceuticals on the market; moreover, the essential properties of this class of drugs remain unclear. It is unknown whether d opioid agonists incur the side effects observed with other opioid drugs, like dependency and constipation. SNC-80 (GSK, NIH) [15] is one of the leading d opioid agonists in development. SNC-80 has a bisarylmethyl substituted piperazine substructure (Fig. 2). Some SNC-80 derivatives for treating pain or urinary incontinence are in clinical development, including DPI-3290 [16] (d and m opioid agonist; GSK, Enhance Biotech), PF-04856880 [17] (ADL-5859; Pfizer, Adolor), PF-04856881 [18] (ADL-5747; Pfizer, Adolor), and a dimethylpiperazine derivative [19] (University of Arizona, NIH) (Fig. 2).

1

Shionogi reports Financial Results for the Third Quarter of Fiscal Year 2009, Shionogi Press Release 2010, February 1.


33

O Et2N

H OMe N

F

N

Me N

N

O N OH DPI-3290 SNC80

O

N

O

N

OH

OH ·HCl O

NH

PF-04856880

·HCl O

NH

PF-04856881

O N

N N

H MeN N

OMe (+)-4-[alpha(R)-[4-Allyl-2(S ),5(R )-dimethylpiperazin-1-yl]3-methoxybenzyl]-N-ethyl-N-methylbenzamide

OH TAN-67

Fig. 2 Structures of d opioid agonists in clinical development

Our group is currently developing TAN-67 (Toray), another lead d opioid agonist with a characteristic 4a-aryl-decahydroisoquinoline structure. We previously discussed the design rationale and pharmacology of this compound [20, 21].

1.3

d Opioid Antagonists

Portoghese and Suzuki have studied the possibility of reducing the undesirable side effects of m opioid agonists by combining them with a d opioid antagonist. This concept is based on the cross-talk among all three types of opioid receptors.

34

H. Nagase and H. Fujii N N

N

OH

F

OH

OH

N O HO

O

O

HO

BNTX

O

N H

·MeSO3H

OH

NTI

TRK-851

Fig. 3 Structures of d opioid antagonists BNTX, NTI, and TRK-851

Portoghese and coworkers have shown that the dependency and tolerance induced by morphine could be treated with 7-benzylidenenaltrexone (BNTX), a d1 opioid antagonist (Fig. 3) [22]. Suzuki and colleagues also suggested that the psychological dependence induced by morphine could be reduced with naltrindole (NTI), another d opioid antagonist (Fig. 3) [23, 24]. However, no drugs of this type are currently in clinical development. It has also been suggested that d opioid antagonists might be potentially useful for immunosuppression [25], but no clinical development has been reported for this indication. The immunosuppressive effect of the d antagonist NTI was suggested to arise from an alternative mechanism [26]. Kamei and colleagues discovered that an opioid network was involved in the cough reflex. A selective d antagonist, TRK-851 (Toray, Fig. 3), is a clinical candidate in development as an antitussive agent [6]. This compound will be discussed in detail in Sect. 2.

1.4

k Opioid Agonists

For the past three decades, considerable effort has been focused on developing an opioid k selective agonist that could eliminate undesirable side effects of morphinelike drugs. In 1982, Upjohn (currently merged with Pfizer) discovered a highly selective k agonist known as U-50,488H [27]. Subsequently, several research groups have modified this structure to create more selective and more potent k agonists (Fig. 4) [29, 30]. In animal models, these k agonists had potent antinociceptive effects and could eliminate the undesirable side effects of morphine-like drugs. They all had very similar structures that included the [N–C–C–N(sp2)] pharmacophore sequence (shaded parts in Fig. 4), because they were derived from U-50,488H. Moreover, they lacked the tyrosine moiety that is essential for opioid activity from the viewpoint of endogenous opioid chemistry (Fig. 5). Currently, the only clinically successful k agonist drug has been TRK-820 (nalfurafine hydrochloride) (Toray, Fig. 6). Almost all of the U-50,488H derivatives apparently had side effects that were different from that of morphine,


Cl

O

Cl N

N N

U-62066 Cl

N N

Cl

Me

U-50,488H

O

O

N

Me

O

Cl

O

O

N N

35

U-69593

O

Cl N

O

O

N

Me

N

S

Me

N

PD-117302

ICI-199441

Me

O

Me

CI-977

Fig. 4 Structures of k agonist U-50,488H and its derivatives. Reprinted from [28], with permission from Elsevier. Copyright (2008) Endomorphin-1(μ) Tyr-Pro-Trp-Phe-NH2 Endomorphin-2(μ) Tyr-Pro-Phe-Phe-NH2 [Met5]enkephalin (δ) Tyr-Gly-Gly-Phe-Met [Leu5]enkephalin(δ) Tyr-Gly-Gly-Phe-Leu Dynorphin A (κ) Tyr-Gly-Gly-Phe-Leu-Arg-Arg-Ile-Arg-Pro-Lys-Leu-Lys-Trp-Asp-Asn-Gln Endogenous Opioids R1 N Tyrosine moiety

HO

R2

O

4,5-epoxymorphinan

Fig. 5 Structures of 4,5-epoxymorphinan and typical opioid peptides. The tyrosine moiety is shown in bold. Reprinted from [28], with permission from Elsevier. Copyright (2008)

H N HO

. HCl

O HO

Fig. 6 Structure of k agonist TRK-820

O H H N CH3 TRK-820

O

36


including dysphoria and psychotomimetic effects. We will describe the design rationale for TRK-820 in Sect. 3.

2 Antitussive d Opioid Antagonist TRK-851 TRK-851 is a clinical candidate for an antitussive drug; it has a novel, complex morphinan ring system. The development of TRK-851 was motivated by the finding that NTI, a selective d opioid receptor antagonist, showed antitussive effect. In this section we will describe the process of developing TRK-851, including the structure–activity relationship (SAR) studies on NTI derivatives and the difficulties encountered in overcoming a defect in the metabolism of a prototype clinical candidate, TRK-850.

2.1

Antitussive Effects of Opioid Ligands

It was recently recognized that opioid receptor agonists have antitussive effects [31]. Morphine (Fig. 7) is a potent m opioid receptor agonist that exhibits marked antitussive effect, and it has been used for the treatment of severe coughs. Codeine (Fig. 7) is one of the most reliable centrally acting antitussive agents; its antitussive

Me

N H

N Me

N O

RO

N

Cl

O

HO

Cl HO

OH

R = H; Morphine R = Me; Codeine

N OH

O

U-50,488H

O

N H

OH

nor-BNI

H3C CH3 O

H2N

S H3C CH3 S O H N N H O

OH O

HN

O

N OH

N H HO

HO DPDPE

O

N H NTI

Fig. 7 Structures of opioid receptor ligands that have antitussive effects (agonists: morphine, codeine, U-50,488H; antagonists: NTI), d opioid agonist DPDPE, and k opioid antagonist nor-BNI

Opioids in Preclinical and Clinical Trials Table 2 Different opioid receptor ligands and their antitussive effects in rats

Opioid ligand (s) m Agonist k Agonist d Agonist m Agonist þ d agonist k Agonist þ d agonist d Antagonist

37 Antitussive effect þþ þ þ

effect is thought to be derived from the activation of both a codeine specific receptor and the m opioid receptor, because one of its active metabolites is morphine. Although these narcotic antitussive agents have sufficient therapeutic effects, they also exhibit m opioid receptor-mediated side effects, e.g., constipation, dependency, and respiratory depression. Therefore, the use of these drugs is limited [31, 32]. The effects of k and d opioid receptor activation on the cough reflex have been particularly investigated by Kamei and coworkers. They showed that U-50,488H (Fig. 7), a selective k opioid receptor agonist, suppressed the cough reflex induced by capsaicin inhalation in rats. This antitussive effect was blocked by coadministration of the k receptor antagonist, nor-BNI (Fig. 7). This suggested that the k opioid receptor had mediated the antitussive effects (Table 2) [33]. On the other hand, the d opioid receptor agonists exhibited activity in contrast to m and k agonists. One d opioid receptor agonist, DPDPE ([D-Pen2, D-Pen5]enkephalin) (Fig. 7), hardly induced antitussive effect, but it blocked the antitussive effect of morphine. The blockade could be antagonized by the coadministration of NTI (Fig. 7), a selective d opioid receptor antagonist (Table 2) [34]. The antitussive activity of U-50,488H was also blocked by DPDPE, and that blockade was also antagonized by the coadministration of NTI [35]. These results suggested that activation of the d opioid receptor suppressed the antitussive activities of m and k opioid agonists. Furthermore, Kamei et al. reported that the administration of NTI alone also produced an antitussive effect (Table 2) [36].

2.2

Mechanism of the Antitussive Effect of NTI

Figure 8 shows the mechanism proposed for the antitussive effect of NTI [36]. When the respiratory tract is stimulated by irritants, e.g., xenobiotics, the cough reflex occurs, and the stress of coughing triggers the secretion of endogenous opioid peptides, which may simultaneously stimulate three types of opioid receptors. In a disease state, the stimulation of the d opioid receptor could block the m and k opioid receptor-mediated suppression of the cough reflex, which would result in continual coughing (Fig. 8a). When a selective d opioid antagonist, e.g., NTI, is administered,

38


Fig. 8 Possible mechanism of the antitussive effect caused by d opioid receptor antagonists. Solid arrows indicate activated pathways; dotted lines indicate inactivated pathways

it blocks the antagonism by d agonist to cough suppression derived from the m and k receptors, and coughing is suppressed (Fig. 8b). These observations strongly supported the feasibility of d opioid receptor antagonists as antitussive drugs that did not have m opioid receptor-mediated side effects.

2.3 2.3.1

Improvement on the Antitussive Activity of NTI Rat Cough Model

In order to discuss the SARs of opioid ligands, we must first describe the rat cough model. In this model, rats are exposed to a capsaicin aerosol that induces coughing (Fig. 9) [36]. The coughs are detected as changes in air pressure inside the rat chamber, and the number of coughs can be counted as the number of peaks on the polygraph with careful distinction of coughs from wriggles. The ED50 values of NTI injected via intraperitoneal (i.p.) and peroral (p.o.) routes were 104 mg/kg and 1,840 mg/kg, respectively, comparable to those of codeine. This cough model was used to determine SARs of NTI derivatives. In addition to the antitussive efficacy of compounds, it is possible to assess not only compounds’ intrinsic affinity for the receptor but their absorption, distribution, metabolism, and excretion (ADME) properties.

2.3.2

Design and Rationale of NTI

Improving a drug’s permeability through the BBB can enhance its concentration in the CNS. Therefore, improving the BBB permeability of NTI might augment its antitussive activity.


39

Fig. 9 Rat model of capsaicin-induced coughing

The BBB consists of microcapillary blood vessels that run throughout the brain in close proximity to brain cells [37, 38]. On the inner surface of the capillaries, the endothelial cells are held tightly together to each other to form a monolayer. Centrally acting drugs must permeate through these endothelial cells to penetrate into the brain. In general, the physicochemical properties of a compound can greatly affect its capacity for passive transcellular BBB permeation. General strategies for modifying drug structures to improve BBB permeability include the following [37, 38]: 1. Reduction of hydrogen bond donors 2. Augmentation of hydrophobicity 3. Reduction of molecular weight In addition to passive transport, active transport can occur with proteins lodged in the BBB that facilitate uptake and efflux of compounds into and out of the CNS. Therefore, compounds can be modified to enhance their affinity for uptake transporters or reduce their affinity for efflux transporters (e.g., P-glycoprotein) to improve BBB permeability [37]. However, these strategies required major structural transformations; thus, we focused on strategies related to changing the physicochemical properties of NTI, as mentioned below.

2.3.3

Introducing Alkyl Substituents into the Indole Ring of NTI

One strategy for improving the BBB permeability of NTI was to augment its hydrophobicity by introducing alkyl substituents into the indole ring. Several NTI derivatives with alkyl substituents were synthesized and tested for potentiation of the antitussive activity. Table 3 presents a comparison of the resulting NTI derivatives. Methylation of the 3-hydroxy group in NTI (compound 1) slightly increased the antitussive activity [39]. In contrast, N-methylation at the 10 position of NTI (compound 2) reduced the antitussive activity. However, simultaneous alkylations (compound 3) at both the 3-hydroxy group and the 10 position markedly improved the antitussive activity. Moreover, introduction of a third alkyl substituent at the 70

40


Table 3 Antitussive effects of alkyl-substituted NTI derivatives in rats (administered i.p.) N

OH

N1' O

R2

7'

R3

3

OR1

Compound R1 R2 R3 ED50 (mg/kg)a cLog P NTI H H H 104 (20.3–553) 3.2 1 Me H H 63.9 (29.4–139) 3.8 2 H Me H 204 (41.8–999) 3.7 3 Me n-Pr H 14.8 (4.09–53.8) 5.3 4 Me Me Me 6.35 (2.05–19.6) 4.8 5 Me Et Me 1.79 (0.67–4.89) 5.3 a ED50 value; the dose that reduces the number of coughs to 50% vs control, expressed as the mean (n ¼ 8). Figures in parentheses indicate 95% confidence limits

position in compounds 4 and 5 led to further enhancement of potency by factors of 16 and 58, respectively, compared to the parent compound. These results suggested that introducing alkyl groups into the indole substructure, especially at the 10 and 70 positions, increased the hydrophobicity of these compounds, which is supported by the calculated log P values (cLog P). This enhanced their BBB permeability and, as a result, the antitussive activities were enhanced. Reducing the number of hydrogen bond donors was another strategy for improving BBB permeability; however, although NTI derivatives 1 and 2 lacked a single hydrogen bond donor, they did not show improved antitussive activity compared to NTI.

2.3.4

NTI Derivatives with an Additional Fused Ring

Among the compounds in Table 3, compound 5 showed the highest antitussive activity; however, it showed a markedly reduced d receptor antagonist activity in the mouse vas deferens (MVD) assay (Table 4). In the MVD assay, the Ke value indicated the potency of the test compound antagonist activity for each receptor type. Compound 5 showed a markedly high Ke value (low potency) for the d receptor compared with that of unmodified NTI [39]. Moreover, compound 5 showed a reduced selectivity for the d receptor over the m and k receptors. This suggested that the antitussive activity of compound 5 may be induced by its active metabolites (e.g., 3-OH analog). We suspected that the bulky substituents in the indole ring of compound 5 might have caused its reduced affinity for the d receptor, which resulted in a reduction in d receptor antagonist activity. Thus, to reduce the bulkiness of the indole ring on

Opioids in Preclinical and Clinical Trials N

41

OH

N reduce steric hinderance

N O

OH

N

Me

O

Me

OMe

OR

5

6-10

Fig. 10 Additional fused ring design of new NTI derivatives

Table 4 Antagonist effect of NTI and its derivatives (MVD assay)a N

OH

N O 3

n

OR 6-10

Compound

R

n

Ke (nM)b Morphine (m) 50 263 17 13

DPDPE (d) U-50,488H (k) NTI – – 0.074 2.9 5 Me – 204 213 6 H 1 0.61 8.6 7 H 2 0.80 28 c 68 8 Me 2 6.3 9 H 3 3.1 18 53 d 10 Me 3 333 17 a DPDPE, morphine, and U-50,488H were used as agonists of d, m, and k opioid receptors, respectively b The Ke values were calculated according to the following equation. Ke ¼ [test compound concentration]/[dose ratio–1]. The dose ratio was the ratio of agonist concentrations that elicited equal responses in the absence and presence of the test compound at increasing concentrations c No antagonism at a test compound concentration of 100 nM d No antagonism at a test compound concentration of 10 nM

compound 5, we designed and synthesized a new type of NTI derivative, with an additional fused ring system (Fig. 10). As expected, the fused ring compounds 6–9 exhibited higher d receptor antagonist activity than compound 5 (Table 4) [39]. In the 3-OMe derivatives, compound 8 (a propylene bridge) showed higher d antagonist potency than compound 10 (a butylene bridge). In the 3-OH derivatives, the d antagonist potency order was 6 (an ethylene bridge) 7 (a propylene bridge) > 9 (a butylene bridge). These results strongly suggested that the d receptor binding affinity was reduced with the

42

H. Nagase and H. Fujii Table 5 Antitussive effects of NTI derivatives that had an additional fused ring structure (administered i.p. in rats) cLog P Compound ED50 (mg/kg)a NTI 104 (20.3–553) 3.2 1,830 (890–3,820)b 6 9.14 (4.23–19.8) 3.8 7 8.58 (2.33–31.7) 4.2 8 (TRK-850) 3.43 (0.93–12.6) 4.8 6.40 (1.54–26.5)b 9 12.3 (3.92–38.3) 4.8 10 9.68 (1.98–47.4) 5.3 a ED50 value; the dose that reduces the number of coughs to 50% vs control, expressed as the mean (n ¼ 8). Figures in parentheses indicate 95% confidence limits b p.o. administration

bulkiness of the indole ring, and a compact fused ring facilitated a higher d receptor binding affinity. Table 5 shows the antitussive activities of the fused ring NTI derivatives [39]. All the fused ring compounds 6–10 showed dramatically improved antitussive activity compared to NTI. In particular, the 3-OMe analog 8, whose methanesulfonic acid salt was selected as a prototype clinical candidate, TRK-850, showed very high antitussive activity; its ED50 value was 30-fold lower than that of NTI. Moreover, when administered p.o., TRK-850 also exhibited a highly potent antitussive effect; its ED50 value was 6.40 mg/kg, or 280 times lower than that of NTI given p.o. The increased hydrophobicity and lower bulkiness of TRK-850 led to potent antitussive effects. In summary, the introduction of alkyl chains into NTI resulted in increased antitussive activities of these compounds, but it simultaneously reduced the d receptor antagonist activity. The introduction of an extra fused ring created a compact-sized hydrophobic moiety attached to the indole ring in NTI. This led to a compound, TRK-850, with both high d receptor antagonism and potent antitussive effects.

2.4 2.4.1

Improvement of Metabolic Defect of TRK-850 Strategies

As mentioned above, TRK-850 had excellent pharmacological properties (potent d antagonist activity and antitussive effect by p.o. administration). However, a serious defect in TRK-850 was observed during the ADME study. After p.o. administration of tritiated TRK-850 in a rat, the plasma sample showed only a small parent peak with HPLC [3H]-detection. This indicated that TRK-850 was

Opioids in Preclinical and Clinical Trials Fig. 11 Structure of compound 8 (TRK-850) and its main metabolic pathways. Reprinted from [6] with permission from Elsevier. Copyright (2008)

43 dealkylation hydroxylation 17

N

OH

hydroxylation

9' 8'

N O

6'

OMe

hydroxylation

8 (TRK-850)

metabolized at an extremely high rate, which later proved to be caused by cytochrome P450 [6]. Therefore, we concluded that TRK-850 was inadequate as a longlasting drug. The metabolism of TRK-850 was investigated in detail by isolating the main metabolites in the plasma samples of rats that received oral TRK-850. We identified four primary metabolites: 17-decyclopropylmethyl, 90 -hydroxy, 80 -hydroxy, and 60 -hydroxy derivatives (Fig. 11) [6]. On the basis of the structures of the four metabolites, we designed strategies for improving the oxidative metabolic profile of TRK-850. In general, oxidative metabolism may be prevented with the following strategies [38]: 1. Incorporating blocking groups at the labile site. 2. Modifying the labile site with bulky substituents, which prohibit the metabolic enzyme from approaching its substrate through steric hindrance. 3. Reducing the lipophilicity of the compound. 4. Introducing electron-withdrawing groups around the labile site to reduce electron density at the labile site and, consequently, prohibit an oxidative metabolic reaction.

2.4.2

Structure–Metabolic Stability Relationships of TRK-850 Derivatives

Several TRK-850 derivatives were synthesized based on the general strategies mentioned above, and their metabolic stabilities were evaluated in the presence of human liver microsomes. Metabolic rates were determined by incubating the compounds with human liver microsomal enzymes and then quantifying the remaining parent peak by HPLC analysis. The data were expressed as the elimination rate constant (KEL) and the relative metabolic rate, or the KEL of the compound compared with the KEL of TRK-850 (KEL ratio). Replacement of one of the metabolic sites, the 60 -methylene group of TRK-850, with an oxygen atom or dimethylmethylene group improved the metabolic stability (Table 6) [6]. The derivatization that most effectively lowered the metabolic rate was the demethylation of the 3-methoxy group (Table 6). The conversion of the 3-methoxy

44


Table 6 Metabolic rates of TRK-850 derivatives in the presence of human liver microsomes N

OH

N X

O

6'

3

OR

Metabolic ratea cLogP (KEL ratio)1 KEL (min ) 8 (TRK-850) CH2 Me 0.341 1 4.8 11 CMe2 Me 0.0618 5.51 5.8 12 O Me 0.0627 5.43 4.2 7 CH2 H 0.0249 13.7 4.2 13 CMe2 H 0.0758 4.5 5.2 14 O H 0.0334 10.2 3.6 a KEL, an elimination rate constant calculated from the time course of compound elimination. KEL ratio ¼ (KEL of a given compound)/(KEL of TRK-850) Compound

X

R

1

group of TRK-850 to a hydroxyl group (compound 7) dramatically improved metabolic stability. This stabilizing effect was also observed in compound 14. These results might be due to the reduced lipophilicity of these compounds, as suggested by the cLog P values. Next, we evaluated the metabolic rates of compound 7 derivatives, which had a substituent in the indole benzene ring (15–28; Table 7) [6]. Introduction of an electron-withdrawing substituent into the indole moiety tended to suppress the metabolism of a compound (16, 18, 19, 21–23, 27). On the other hand, an electron-donating substituent tended to enhance the metabolism of a compound (20, 26, 28). These observations suggested that the electron density around the indole moiety was correlated to the cytochrome P450 oxidative metabolic rate. In addition, the position of substitution was important. For example, blocking at the 80 -position suppressed the metabolism more effectively than blocking at the 70 - or 90 -positions.

2.4.3

Antitussive Activities of TRK-850 Derivatives: Journey to TRK-851

The antitussive effects of selected TRK-850 derivatives were evaluated (Table 8) [6]. Incorporation of a substituent into the indole moiety resulted in lower potency than that of compound 7; nevertheless, some derivatives still exhibited high antitussive activity (19–21, 24, 26). Incorporation of bulky substituents at the 80 -position also tended to reduce potency (22, 23). Incorporation of fluorine, the smallest substituent with the greatest electron-withdrawing capacity, resulted in the


45

Table 7 Metabolic rates of compound 7 derivatives in the presence of human liver microsomes N

OH

9' 8'

R N

7'

O OH

Compound

Metabolic ratea

R 1

KEL (min ) (KEL ratio)1 7 H 0.0249 13.7 15 70 -Cl 0.0290 11.7 0.0218 15.6 16 70 -Br 17 70 -OMe 0.0262 13.0 0.0177 19.2 18 80 -Cl 19 80 -Br 0.0151 22.6 20 80 -OMe 0.0331 10.3 0.0174 19.6 21 80 -F 22 80 -OCF3 0.00909 37.5 23 80 -CF3 0.0162 21.0 0.0270 12.6 24 90 -Cl 25 90 -Br 0.0371 9.20 0.0285 12.0 26 90 -OMe 27 90 -CF3 0.0167 20.4 0.0339 10.1 28 90 -Me a KEL, an elimination rate constant calculated from the time course of compound elimination. KEL ratio ¼ (KEL of a given compound)/(KEL of TRK-850)

minimum loss of potency (21). Among these derivatives, compound 21 showed the most potent antitussive activity and had a metabolic rate 20 times lower than that of the parent compound, TRK-850. The methanesulfonic acid salt of compound 21 was called TRK-851. Figure 12 shows that p.o. administration of TRK-851 reduced the number of coughs in a dose-dependent manner with an ED50 value of 35.5 mg/ kg. Thus, the potency of TRK-851 was over 50-fold higher than that of the starting compound, NTI [6]. The opioid receptor selectivity of TRK-851 was tested in the MVD assay (Table 9). TRK-851 strongly antagonized the agonist activity of DPDPE (d), and it showed over 200 times greater selectivity for the d opioid receptor than for m or k receptors. These results strongly suggested that the antitussive effect of TRK-851 was derived from its marked antagonism of d opioid receptors [6].

2.5

Summary

NTI derivatives that possessed hydrophobic substituents were designed and synthesized based on the hypothesis that increased hydrophobicity might improve their

46


Table 8 Antitussive effects of TRK-850 derivatives in rats (administered s.c.) N

OH

9' 8'

R2 7'

N O

X

OR1

ED50 (mg/kg)a 0.20 (0.06–0.6) 8.58 (2.33–31.7)b 8 (TRK-850) CH2 Me H 3.43 (0.93–12.6)b 0 15 CH2 H 7 -Cl 13.8 (0.62–310) 16 CH2 H 70 -Br 27.4 (8.81–85.1) H 70 -OMe 54.2 (18.6–158) 17 CH2 18 CH2 H 80 -Cl 78.4 (11.5–538)b 0 19 CH2 H 8 -Br 5.05 (2.58–9.86) 20 CH2 H 80 -OMe 2.0 (0.4–9.4) 21 (TRK-851) CH2 H 80 -F 1.7 (0.4–7.3) 15.7 (5.31–46.4)b 22 CH2 H 80 -OCF3 >1,000 23 CH2 H 80 -CF3 >1,000 24 CH2 H 90 -Cl 3.5 (1.4–8.7) H 90 -Br 94.6 (5.11–389) 25 CH2 26 CH2 H 90 -OMe 8.67 (1.02–73.6) a ED50 value; the dose that reduces the number of coughs to 50% vs control, expressed as the mean (n = 8). Figures in parentheses indicate 95% confidence limits b Administered i.p. Compound 7

X CH2

R1 H

R2 H

permeability through the BBB. This was expected to increase their antitussive activities compared with that of unmodified NTI. Although the introduction of alkyl groups into the NTI skeleton increased the antitussive activities of these derivatives, it simultaneously decreased the d opioid receptor antagonist activity. Thus, the antitussive activities of these compounds may be affected more potently by brain exposure of the compounds rather than by their d antagonist activity. The introduction of an extra fused ring, a compact-sized moiety, into the indole ring of NTI conferred both suitable hydrophobicity and low steric hindrance. Consequently, compound 8 (TRK-850) was identified as a prototype clinical candidate that exhibited both potent antitussive activity and selective d receptor antagonist activity. However, TRK-850 was found to be susceptible to oxidative metabolism by cytochrome P450. The results from structure optimization studies suggested that a drug candidate must have sufficiently low hydrophobicity to prevent a high metabolic rate, but also have sufficiently high hydrophobicity to retain potent antitussive activity. Therefore, additional compact indole substituents that were expected to


47

Fig. 12 Antitussive effect of compound 21 (TRK-851) on capsaicin-induced coughing in rats (p.o. administration). The number of coughs over 3 min were counted in the same rats before and 60 min after p.o. administration. The ED 50 value (the dose that reduced the number of coughs to 50% vs control) for the inhibition of coughing was calculated to be 35.5 mg/kg (95% confidence limits: 16.8–72.9 mg/kg). Reprinted from [6] with permission from Elsevier. Copyright (2008)

Table 9 Compound 21 (TRK-851) antagonism of opioid receptors (MVD assay)a Compound Ke (nM)b DPDPE (d) Morphine (m) U-50,488H (k) c 21 (TRK-851) 1.46 333 a DPDPE, morphine, and U-50,488H were used as agonists of d, m, and k opioid receptors, respectively b The Ke values were calculated according to the following equation: Ke ¼ [test compound concentration]/[dose ratio–1]. The dose ratio was the ratio of agonist concentrations that elicited equal responses in the absence and presence of the test compound at increasing concentrations c No antagonism at test compound concentration of 10 nM

metabolize slowly were examined for their antitussive activity. Modifications of TRK-850, including demethylation of the 3-methoxy group and introduction of 80 -fluorine, provided the appropriate hydrophobicity and potent d antagonist activity. Finally, TRK-851, a potent, selective d receptor antagonist that metabolized slowly, was identified as a highly potent oral antitussive agent, and was chosen for further clinical evaluation.

48


3 Antipruritic k Opioid Agonist TRK-820 (Nalfurafine Hydrochloride) Nalfurafine hydrochloride is a first-in-class, nonaddictive opioid drug that is used as an antipruritic for severe itching associated with hemodialysis. This drug was launched in Japan in 2009. It has a unique morphinan structure that is rarely observed in typical and traditional opioid k agonists. The design rationale for this characteristic compound was described in a previous study [7, 28], and would be of interest to researchers in the opioid field. We will summarize the rationale in Sect. 4.

3.1

Design Rationale

Since the Upjohn Group first described the synthesis of U-50,488H (Fig. 4) [27], numerous U-50,488H derivatives have been synthesized and examined for pharmacological effects. However, none of these drugs was appropriate for practical indications. We questioned whether U-50,488H derivatives were actually k opioid agonists. Therefore, we designed and synthesized a new type of nonpeptide k agonist with a novel chemical structure. This compound retained the tyrosine moiety in order to reduce the side effects of the prototype k agonists, including dysphoria and psychotomimetic effects [7, 40]. Portoghese and colleagues at Minnesota University proposed the “MessageAddress Concept” for designing selective d and k antagonists (Fig. 13: NTI and nor-BNI) [41, 42]. They pointed out that the 4,5-epoxymorphinan skeleton, defined as the “message” subsite, which is commonly found in NTI and nor-BNI, was necessary for the intrinsic activity of an opioid. The other part of the structure was defined as the “address” subsite, and it is involved in the receptor type selectivity. As shown in Fig. 13, the receptor type selectivity of these opioid antagonists can be regulated by altering the size of the address structure. Although this concept was suitable for designing a selective opioid antagonist, a further strategy was necessary to design an opioid agonist. The strategy for designing an opioid agonist included a working hypothesis of an “accessory site”. This hypothesis holds that the receptor can change its shape to accommodate the structure of an agonist (“induced fit”) and facilitate agonist binding to the receptor. This change would lead to activating the signal transduction pathway that expressed the effects of the agonist. On the other hand, the antagonist might be considered to be a compound with an extra structural part that interferes with the accommodating changes in the receptor. As a result, the antagonist shows no agonistic effects. The structural moiety that could interfere with the induced fit is called an "accessory site” [43], and it comprises a highly hydrophobic moiety and a sterically hindered structure.

Opioids in Preclinical and Clinical Trials Fig. 13 Message-Address Concept proposed for opioid antagonists. Reprinted from [28] with permission from Elsevier. Copyright (2008)

49 OH N O

O

μ: Naltrexone OH OH N N O H δ: NTI

OH OH

HO

N

κ: nor-BNI

N N O H O OH HO

Message subsite

Address subsite

To design a new agonist, we first attempted to remove the accessory site of norBNI and maintain the message and address sites. The message site of nor-BNI, a 4,5-epoxymorphinan skeleton with a cyclopropylmethyl substituent, was considered to be indispensable for opioid activity because it corresponded to the tyrosine of endogenous opioid peptides. Therefore, we postulated that the accessory site of nor-BNI was located in the address subsite of this antagonist. The meso isomer of nor-BNI, 27 (Fig. 14), showed k selectivity and antagonist activity similar to those of nor-BNI [44]. Compound 28, which had a nor-BNI address site, but lacked the phenol ring, also maintained high k receptor selectivity, similar to that of nor-BNI [45]. These facts indicated that neither the phenol ring nor the hydroxyl group of the ring junction in the address site of nor-BNI were required for k selectivity or antagonist activity. Based on a comparison between the k antagonist 28, naltrexone, and NTI, we proposed that the feature of the address subsite that was essential for receptor selectivity was the length of the address site (Fig. 14). From this point of view, the design of a k selective agonist required the removal of the accessory site from compound 28, but the length of the address site must be maintained for binding the k receptor. On the basis of that concept, we designed and synthesized compounds (A) that had a C6 (carbon atom in the 6-position) and an X (a hetero atom) with a single bond (Fig. 14). The single bond was expected to give structural flexibility to the ligand, which would facilitate the ability of the receptor to assume an induced fit for binding the agonist. Based on these design principles, a number of compounds were synthesized (29–33) with the proper side chain structure, size, and length (address subsite) for a k agonist activity.

50

H. Nagase and H. Fujii HO OH

O

N O

N H

N

OH N

N H

OH O

OH nor-BNI OH

OH 28 (non-Phenol Form) (κ-antagonist)

HO

N

N O

N H

minimize lipophilc part

O

OH N

OH

N H

HO

27 (meso isomer)

O

6 X

A OH Basic Design

Fig. 14 Basic design of a new chemical class opioid k agonist. Reprinted from [28] with permission from Elsevier. Copyright (2008)

3.2

Pharmacology and Medicinal Chemistry

Table 10 shows the results of screening for the most appropriate X spacer for k agonist activity. The side chain was fixed at a 6-carbon length, which is similar to the size of the address site of nor-BNI. Opioid receptor selectivity and agonistic activity were evaluated with the MVD assay [46]. The agonist potencies were expressed as IC50 values. The Ke value indicated the receptor selectivity of test agonists. After detecting the opioid receptor agonist activity of test compounds in vitro, the acetic acid writhing (AAW) test [47] was conducted in mice to estimate the in vivo potency. The analgesic effects of test compounds could be effectively antagonized by pretreatment with the opioid k antagonist, nor-BNI. Almost all the compounds showed similar agonistic activity in the MVD assay. Although KT-90 and KT-95 (Fig. 15) [48–50] which had thioester structures, reportedly showed opioid k agonist activity, compound 31, also with a thioester, showed almost no agonistic activity in vitro or in vivo. This finding revealed the importance of the chemical structure of the “address” site of the analog. Amide compounds, particularly the N-methyl amido compound 32b, showed the most potent agonistic activity in the AAW test and the highest k selectivity in the MVD assay. Table 11 shows that both k agonistic activity and selectivity were affected by the introduction of an unsaturated bond and a lipophilic moiety into the N-methyl amide side chain. Often, the binding affinity or agonistic activity could be increased by introducing an unsaturated bond into the side chain. This might fix a favorable


51

Table 10 SAR studies on the variation of the spacer X

H N HO

HO

O H H

X

R

2

MVDa AAW, ED50 (mg/kg)j b IC50 (nM) Ke (nM) s.c. p.o. p.o./s.c. de kh mc 29 O-C6H13 16 4.5 1.5 0.12 1.8 NDk NDk 30 S-C6H13 38% inhibition (1 mM) 16 NDk NDk 31 S(CO)-C5H11 Not effective Not effective 2.0 14 NCf 0.3 0.29 NDk NDk 32a NH(CO)-C5H11 33 NH-C6H13 25 4.4 2.3 0.26 25% inhibition (10 mg/kg) 32b NMe(CO)-C5H11 15 UDd 90g 0.046i 0.77 0.63 8.2 a Data were expressed as the mean (n = 4–9) b The Ke values were calculated from the following equation: Ke = [antagonist]/(IC50 ratio–1); [antagonist]: concentration of the antagonist; IC50 ratio:the IC50 without the antagonist over the IC50 with the antagonist. Each preparation was incubated with a selective antagonist for 20 min before the addition of a test compound c Naloxone (30 nM) was used as a m selective antagonist d Undetectable (IC50 did not change with up to 100 nM of naltrexone) e NTI (10 nM) was used as a d selective antagonist f Not calculated (the IC50 ratio was too small to calculate the Ke value) g NTI (100 nM) was used h nor-BNI (10 nM) was used as a k selective antagonist i nor-BNI (20 nM) was used j Data were expressed as the mean (n = 10) k Not determined Reprinted from [28] with permission from Elsevier. Copyright (2008) Compound

X-R2

O

N H

AcO

Fig. 15 Structures of KT-90 and KT-95

O

KT-90

N H

SAc

AcO

O

SAc

KT-95

conformation (Fig. 16) and may contribute to p–p interactions with the receptor. We also attempted to optimize the length of the side chain. In the course of these SAR studies, we found an attractive lead compound, 32f (Fig. 16; Table 11),

52

H. Nagase and H. Fujii R2

R2

32c

Me

32l

N 32d

HO

HO

O

R2 6 N CH3

F

32m

O 32e

NO2

32n 32f 32o Cl 32g

Cl

32p

S

32h F3 C 32i

32q CF3

S 32r

O

32j CF3 32k

OMe

32s O

Fig. 16 Potential k agonist structures: 6-amide derivatives of compound 32b. Reprinted from [28] with permission from Elsevier. Copyright (2008)

a 6b-cinnamamido derivative. Compound 32f showed high selectivity for the k receptor in the MVD assay and significant in vivo k agonist potency in the AAW test, even with p.o. administration. Furthermore, the agonist effect of 32f showed adequate duration in the in vivo test. However, 32f had significant side effects. Therefore, a lead optimization study was performed by focusing on the stereochemistry, length, and aromatic ring structure of the side chain. A number of 32f derivatives were synthesized by condensing a- and b-N-methylnaltrexamine with various carboxylic acids (Fig. 16; Table 12). Compounds a- and b-32 g were considered the simple combination of a morphinan structure with U-50,488H, because U-50,488H had the same side chain (3,4-dichlorophenylacetamido). However, their analgesic activities, especially when administered p.o., were neither potent nor long lasting. Both the compounds that were simple derivations of the


53

Table 11 SAR study on the variation of the side chain R2 (structures shown in Fig. 16) AAW, ED50 (mg/kg)i Compound MVDa b IC50 (nM) Ke (nM) s.c. p.o. p.o./s.c. Durationj c e g m d k 32b 15 UDd 89.6f 0.046h 0.77 0.63 8.2 NDk 32c 0.65 1,797 UDd 0.072 0.024 0.21 8.8 S 32d 0.46 5,713 143 1.08 0.004 0.071 17.0 L 32e 0.019 UDd 188 0.008 0.040 0.29 7.3 S 32f 0.17 922 263 0.861 0.002 0.011 5.5 L a Data were expressed as the mean (n ¼ 4–9) b The Ke values were calculated from the following equation: Ke ¼ [antagonist]/(IC50 ratio–1); [antagonist]: concentration of the antagonist; IC50 ratio: IC50 without the antagonist over the IC50 with the antagonist. Each preparation was incubated with a selective antagonist for 20 min before the addition of a test compound c Naloxone (30 nM) was used as a m selective antagonist d Undetectable e NTI (10 nM) was used as a d selective antagonist f NTI (100 nM) was used g nor-BNI (10 nM) was used as a k selective antagonist h nor-BNI (20 nM) was used i Data were expressed as the mean (n ¼ 10) j Dose that inhibited 90% of the AAW behavior at 30 min after test compound administration (s.c.). AAW test was performed at time points of 60, 120, 180, and 240 min postadministration. S (Short): effect decreased within 120 min; L (Long): effect continued for 180 min or more k Not determined Reprinted from [28] with permission from Elsevier. Copyright (2008)

U-50,488H chemical structure proved to be unsuccessful. In the course of the lead optimization process, we found the two following tendencies: (1) compounds with a 6b-amido structure showed a low s.c./p.o. potency ratio and a long p.o. duration in the AAW test and (2) the aromatic ring structure in the C6-side chain was one of the most important components for optimizing the pharmacological profile, particularly for attenuating the psychotomimetic effect. Finally, we found that the compound with a furanacrylamido side chain, b-32s, had the best properties. Thus, the pharmacological profile of compound b-32s, or TRK-820, was examined in detail. The agonist potency of b-32s was 6,000-fold and 1,800-fold stronger than that of morphine in the guinea pig ileum (GPI) and MVD preparations, respectively (Table 13) [28]. Compared to U-50,488H, TRK-820 was approximately 140 times and 30 times more potent in the GPI and MVD assays, respectively. We also evaluated TRK-820 for its antinociceptive effects in the AAW and tail flick (TF) tests in mice. TRK-820 showed high antinociceptive activity in both tests (Table 13) [28], with ED50 values of 0.0033 mg/kg (AAW) and 0.062 mg/kg (TF). These activities were 85–175 times more potent than that of morphine and 80–350 times more potent than that of U-50,488H. This analgesic effect was antagonized by nor-BNI (k antagonist) but not by NTI (d antagonist) or by low-dose naloxone (m antagonist). Furthermore, TRK-820 was notable for its low psychotomimetic effects. In contrast, morphine showed a significant preferential (addictive) effect

54


Table 12 SAR study on the variation of the side chain configuration and R2 (structures shown in Fig. 16) AAW, ED50 (mg/kg)h Compound 6-a or 6-b MVDa c IC50 (nM) Ke (nM) s.c. p.o. p.o./s.c. Durationi d f g m d k 32f b 0.166 922 263 0.861 0.002 0.011 5.5 L a 0.12 16.5 0.43 4.90 0.0058 0.23 39.3 L 32g b 17.0 17,000 1,113 1.37 0.46 17.99 39.1 NDb a 0.40 53.0 17.1 0.55 0.017 0.83 48.0 S 32h b NDb – – – 0.18 NDb – NDb e 32i b 0.47 UD 291 3.20 0.0046 0.034 7.4 NDb 32j b 0.07 524 78.0 0.20 0.023 0.026 1.1 L 32k b 0.75 60.9 96.4 1.60 0.0011 0.0048 4.4 L 32l b 0.32 1,778 64.5 1.95 0.0049 0.064 9.7 L 32m b 1.60 UDe 276 2.37 0.0019 0.0070 3.7 L 32n b 0.23 185 89.0 1.40 0.017 0.220 12.9 NDb b b b b b 32o b ND ND ND ND 3.6 ND – NDb 32p b 0.10 UDe 89.6 0.28 0.0027 0.026 9.6 L UDe 1.71 0.0042 0.040 9.5 L 32q b 0.10 UDe 32r b 0.55 UDe UDe 0.20 0.0016 0.015 9.4 L 32s a 0.049 31.9 245 0.91 0.0081 0.13 16.0 NDb b 0.42 14,000 41.6 0.16 0.0033 0.032 9.7 L a Data were expressed as the mean (n ¼ 4–9) b Not determined c The Ke values were calculated from the following equation: Ke = [antagonist]/(IC50 ratio–1). [antagonist]: concentration of the antagonist; IC50 ratio:IC50 without the antagonist over IC50 with the antagonist. Each preparation was incubated with a selective antagonist for 20 min before the addition of a test compound d Naloxone (30 nM) was used as a m selective antagonist e Undetectable f NTI (10 nM) was used as a d selective antagonist g nor-BNI (10 nM) was used as a k selective antagonist h Data were expressed as the mean (n ¼ 10) i Dose that could inhibit 90% of the AAW behavior at 30 min after test compound administration (s.c.). AAW test was performed at time points of 60, 120, 180, and 240 min postadministration. S (Short): effect decreased within 120 min; L (Long): effect continued for 180 min or more Reprinted from [28] with permission from Elsevier. Copyright (2008)

and U-50,488H appeared to induce an aversive (dysphoric) behavior in test animals. It was concluded that TRK-820 had a neutral effect (neither preferential nor aversive effect) on behavior and, thus, it was expected to cause less psychotomimetic side effects [25]. Next, we attempted to clarify which part of the TRK-820 structure was associated with its excellent pharmacological profile. The AAW test (s.c.) in mice was selected to evaluate structure–activity and structure–ADME relationships. Table 14 shows a summary of the results of a SAR study on compound TRK-820 derivatives (34–42). Although the main factor that determined activity on the AAW test remains to be clarified, these data implied the importance of the followings: (1) the tyrosine components (17-nitrogen and the 3-hydroxy group) were indispensable for the

Table 13 Detailed comparison of compound TRK-820 (b-32s) with morphine and U-5,0488H Compound GPI MVD AAW (s.c.) TF (s.c.) ED50 ED50 IC50a (nM) Ke (nM)b m/k IC50a (nM) Ke (nM)b m/k d/k (mg/kg)a (mg/kg)a k m k m d b-32s 0.0081 0.070c 5.5f 78.6 0.080 0.049c 48f NCi 980 – 0.0033 0.062 d g d g 5.06 0.25 145.1 53.5 3.29 7.35h 0.06 0.14 0.58 5.26 Morphine 49.3 20.5 U-50,488H 1.12 0.031c 12.1f 390 2.35 0.031e 31.5f 41.2h 1016 1329 1.16 5.18 a Data were expressed as the mean (n = 5–7) b The Ke values were calculated from the following equation: Ke = [antagonist]/(IC50 ratio–1). [antagonist]: concentration of the antagonist; IC50 ratio: IC50 without the antagonist over IC50 with the antagonist. Each preparation was incubated with a selective antagonist for 20 min before the addition of a test compound c nor-BNI (10 nM) was used as a k selective antagonist d nor-BNI (100 nM) was used as a k selective antagonist e nor-BNI (1 nM) was used as a k selective antagonist f Naloxone (100 nM) was used as a m selective antagonist g Naloxone (20 nM) was used as a m selective antagonist h NTI (100 nM) was used as a d selective antagonist i Not calculated Reprinted from [40] with permission from Elsevier. Copyright (2008)

Opioids in Preclinical and Clinical Trials 55

56


Table 14 SAR study on compound TRK-820 derivatives R1 H N R4 O O H H X

R3

O

Compound R1

R3

R4

MVDa AAW, ED50 (mg/kg)h IC50 (nM) Ke (nM)c md df kg TRK-820 CPM OH OH NMe 0.42 14,000 41.6 0.16 0.0033 34 Me OH OH NMe 55 8.1 33 6.3 1.03 – – – 7.07 35 H OH OH NMe NDb 36 Phenethyl OH OH NMe NDb – – – 1.03 37 CPM H OH NMe 27.2 12.0 8.40 0.07 0.013 38 CPM NH2 OH NMe NDb – – – 0.20 39 CPM NHAc OH NMe NDb – – – >10 40 CPM OH H NMe 0.035 8.04 1.31 0.07 0.02 41 CPM OH OH NH 0.87 UDe 8,100 0.44 0.95 42 CPM OH OH S NDb – – – >10 a Data were expressed as the mean (n ¼ 10) b Not determined c The Ke values were calculated with the following equation: Ke ¼ [antagonist]/(IC50 ratio–1). [antagonist]: concentration of the antagonist; IC50 ratio: IC50 without the antagonist over IC50 with the antagonist. Each preparation was incubated with a selective antagonist for 20 min before the addition of a test compound d Naloxone (30 nM) was used as a m selective antagonist e Undetectable f NTI (10 nM) was used as a d selective antagonist g nor-BNI (10 nM) was used as a k selective antagonist h Data were expressed as the mean (n ¼ 4–9) Reprinted from [28] with permission from Elsevier. Copyright (2008) X

in vivo activity of TRK-820 and (2) the 17-cyclopropylmethyl, 14-hydroxy, and Nmethyl substituents were also important for its agonist activity.

3.3

Clinical Development of TRK-820 in Postoperative Surgery

TRK-820 was first developed as an analgesic for postoperative pain. However, in a Phase II clinical trial, it was concluded that, although the analgesic effect could be sufficiently potent, the safety margin was found to be insufficient. During that study, a physician suggested that patients who received a m opioid receptor


57

agonist, like morphine often complained of an intolerable itch; in contrast, patients who were treated with TRK-820 rarely complained of itching. In addition to this pharmacological observation, m opioid agonists and k opioid agonists often showed contrary effects; for example, a preference for an aversive stimulus or hyperactivity in response to sedation. These findings led us to estimate the antipruritic effect of TRK-820. After confirming a significant antipruritic effect of TRK-820 in various animal itching models established by Kuraishi et al. [51–58], we initiated another clinical development study with the intent to develop an antipruritic agent.

3.4 3.4.1

Clinical Development of TRK-820 as an Antipruritic Agent Antipruritic Activity of TRK-820 in an Animal Model

Epidural or intrathecal (i.t.) administration of morphine produced an itching sensation in humans [59, 60]. It has been reported that intradermal (i.d.) injections of histamine or substance P and intracisternal (i.c.) injections of morphine induced scratching in mice [61–63]. Scratching behavior in mice is considered to represent the itching sensation observed in humans. The administration of TRK-820 (p.o.) inhibited the scratching behavior in mice induced by histamine (i.d.), with an ED50 of 7.3 mg/kg (Fig. 17a) [51, 54, 57]. Moreover, TRK-820 (p.o.) inhibited substance P (i.d.) induced scratching behavior in mice (ED50 ¼ 19.6 mg/kg, p.o.), which was resistant to treatment with antihistamine drugs (chlorpheniramine and ketotifen) (Fig. 17b) [51, 54, 57]. The antipruritic effect of TRK-820 was antagonized by norBNI (Fig. 18) [51, 54]. Based on these studies, TRK-820 was expected to be a more effective antipruritic than antihistamines. Furthermore, TRK-820 inhibited morphine (i.c.) induced scratching behavior in mice, but ketotifen did not [52, 54, 57]. This indicated that TRK-820 might also have an antipruritic effect on antihistamine-resistant pruritus of CNS origin. In addition, scratching behavior in monkeys induced by morphine (i.v. or i.t. injections) could be attenuated by i.v. or intramuscular injections of TRK-820 [53, 58]. TRK-820 was also reported to exhibit antipruritic effects on scratching induced by compound 48/40 [56] or chloroquine [55], and itching observed in disease models for autoimmune disease [64] or cholestasis induced by chronic ethynylestradiol injections [65]. 3.4.2

Clinical Effectiveness on Uremic Pruritus

Patients on hemodialysis often complain of an intractable pruritus (uremic pruritus). Indeed, the consequent sleep disorder can impair the quality of life. Clinical and preclinical data suggested that the m opioid system induced itching, but not the k system. The antipruritic efficacy of nalfurafine hydrochloride (general name of TRK-820) was demonstrated for patients on hemodialysis in two multicenter, randomized, double-blind, placebo-controlled, clinical studies in Europe [66].

58

H. Nagase and H. Fujii b 150

150

125

125

100

100

% of Control

% of Control

a

75 50

75 50

25

25

0

0 0

0

3 10 TRK-820 (μg/kg, p.o.)

30

100

0

0

3 10 TRK-820 (μg/kg, p.o.)

30

100

Fig. 17 Effects of TRK-820 on the scratching behavior in mice induced by histamine (a) or substance P (b). Mice (ICR strain) were given a p.o. administration of vehicle (open bars) or TRK820 (filled bars). Then, 30 min later, phosphate-buffered saline, pH 7.4 (PBS), histamine (a), or substance P (b) was i.d.-injected. Immediately after the injection of PBS, histamine, or substance P, the number of scratching events was recorded and counted over a 30-min period. The numbers of scratching events observed with histamine or substance P injections in the absence of TRK-820 were considered 100% on the vertical axes. Each value represents the mean standard error of the mean (S.E.M.) (n ¼ 8). **P < 0.01 compared between the PBS and histamine- or substance P-injected groups (Dunnett’s test). Reprinted from [51] with permission from Elsevier. Copyright (2002)

Recently, Kumagai and colleagues also published results that clearly showed that nalfurafine hydrochloride was an effective treatment for itching in humans [67, 68]. The effects of the drug on severe itching in patients on hemodialysis was evaluated in a Phase III, randomized, double-blind, placebo-controlled study in Japan. In that study, the efficacy and safety of nalfurafine hydrochloride were prospectively investigated in 337 patients on hemodialysis with itching that was resistant to currently available treatments, e.g., antihistamines. They randomly (1:1:1) administered 5 mg (high dose) or 2.5 mg (low dose) of the drug or a placebo orally for 14 days in a double-blind study design. The primary endpoint was the mean decrease in the visual analog scale (VAS)2 from baseline. Decreases in the VAS were significantly larger in both the high dose (n ¼ 114) and the low dose (n ¼ 112) groups compared to the placebo group (n ¼ 111, P ¼ 0.0002 and P ¼ 0.0001, respectively; one-sided test at 2.5% significance level; Fig. 19). The incidence of adverse drug reactions (ADRs) was 35.1%, 25.0%, and 16.2% in the

2

The VAS test consisted of a 100-mm horizontal line without scale markings. The patients were asked to mark the intensity of itching on the scale, where the right end of the line (100 mm) indicated the strongest possible itching and the left end (0 mm) indicated no itching. The VAS has been widely accepted as a quantitative index of subjective sensations like pain and itching.


59

150

% of Control

125 100 75 50 25

none

PBS

0

1

3

10

nor-BNI (mg/kg, s.c.) TRK-820 (100 μg/ kg, p.o.)

Substance P

Fig. 18 Effect of nor-BNI on the antipruritic effect of TRK-820. Mice (ICR strain) were given an s.c. injection of nor-BNI or PBS (hatched bar). The next day, mice were given a p.o. administration of 100 mg/kg of TRK-820 (filled bars) or vehicle (open bars), and 30 min later, either PBS (hatched bar) or substance P was i.d. injected. Immediately after the latter, the number of scratching events was recorded and counted over a 30-min period. The number of scratching events recorded with substance P in the absence of nor-BNI and TRK-820 was considered to be 100% on the vertical axis. Each value represents the mean S.E.M. (n ¼ 8). *P < 0.05, **P < 0.01 when compared with the corresponding vehicle-injected group (unpaired t-test). Reprinted from [51] with permission from Elsevier. Copyright (2002)

Fig. 19 Changes in VAS values from the preobservation period. All symbols show the mean values of VAS changes. *P < 0.025 compared with the placebo group, one-sided ANCOVA

60


high dose, low dose, and placebo groups, respectively. Mild to moderate ADRs were observed in 10 of the 226 patients, and the most common was insomnia (sleep disturbance), observed in 24 of the 226 patients. However, because all the ADRs were transient and readily resolved, nalfurafine hydrochloride could be considered a safe agent. This Phase III study clearly showed that oral nalfurafine hydrochloride effectively reduced uremic pruritus with few significant ADRs. After the Phase III study, a 1-year open-label study showed that nalfurafine hydrochloride (5 mg, p.o.) provided antipruritic effects for 211 patients on hemodialysis with itching that was resistant to currently available treatments. These results suggested that tolerance to the antipruritic effect of the agent was not observed, at least after 1 year of treatment. It is noteworthy that nalfurafine hydrochloride exhibited neither physical nor psychological dependency [68]. This novel drug was officially approved for clinical use in January 2009 by the Ministry of Health, Labour, and Welfare of Japan. This is the first pharmaceutical success ever achieved for a selective opioid k agonist. All the other compounds derived from modifications of U-50,488H have been withdrawn or halted in early clinical phase development. Furthermore, nalfurafine hydrochloride is recognized as the first nonnarcotic opioid drug in history.

3.5

Concluding Remarks

The development of opioid drugs has been considered a high-risk endeavor. Indeed, it was expected to be particularly tough because the goal was to discover an opioid drug without addictive properties. Because opioids have potent analgesic, antitussive, antipruritic, and antiurinary incontinence effects, much more research is expected to be undertaken in order to produce many ideal drugs. The success of TRK-820 will contribute to the initiation of new and creative studies for discovering clinically applicable opioid compounds.

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Synthesis of 14-Alkoxymorphinan Derivatives and Their Pharmacological Actions Helmut Schmidhammer and Mariana Spetea

Abstract Among opioids, morphinans play an important role as therapeutically valuable drugs. They include pain relieving agents such as naturally occurring alkaloids (e.g. morphine, codeine), semisynthetic derivatives (e.g. oxycodone, oxymorphone, buprenorphine), and synthetic analogs (e.g. levorphanol). Currently used opioid analgesics also share a number of severe side effects, limiting their clinical usefulness. The antagonist morphinans, naloxone and naltrexone are used to treat opioid overdose, opioid dependence, and alcoholism. All these opioid drugs produce their biological actions through three receptor types, m, d, and k, belonging to the G-protein-coupled receptor family. Considerable effort has been put forward to understand the appropriate use of opioid analgesics, while medicinal chemistry and opioid pharmacology have been continuously engaged in the search for safer, more efficacious and nonaddicting opioid compounds, with the final goal to reduce complications and to improve patient compliance. Toward this goal, recent advances in chemistry, ligand-based structure activity relationships and pharmacology of 14-alkoxymorphinans are reviewed in this chapter. Current developments of different structural patterns of 14-alkoxymorphinans as research tools and their potential therapeutic opportunities are also summarized. Keywords 14-Alkoxymorphinans Opioid agonist Opioid antagonist Opioid receptors Opioids

H. Schmidhammer (*) and M. Spetea Department of Pharmaceutical Chemistry, Institute of Pharmacy and Center for Molecular Biosciences Innsbruck (CMBI), University of Innsbruck, Innrain 52a, 6020 Innsbruck, Austria e-mail: [email protected]

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H. Schmidhammer and M. Spetea

Contents 1 2

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 2.1 Synthesis of 14-Alkoxymorphinan-6-ones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 2.2 Synthesis of 14-Alkoxy-Substituted Indolo- and Benzofuromorphinans . . . . . . . . . . . . . 70 2.3 Introduction of New Functionalities to 14-Alkoxymorphinan-6-ones . . . . . . . . . . . . . . . 71 3 Pharmacological Actions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 3.1 14-Alkoxymorphinan-6-ones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 3.2 6-Cyanomorphinans . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 3.3 6-Amino Acid-Substituted 14-Alkoxymorphinans . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 3.4 14-Alkoxy-Substituted Indolo- and Benzofuromorphinans . . . . . . . . . . . . . . . . . . . . . . . . . . 82 4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85

Abbreviations [35S]GTPgS AD50 BBB CBE CHO CNS DMF ED50 GPI HP IC50 IL-2 Ki MeOH MLR MVD NaH NTB NTI PBMC PPOM PPQ RVD SAR s.c. TF TosMIC

Guanosine-50 -O-(3-[35S]thio)-triphosphate Analgesic dose necessary to elicit a 50% effect Blood–brain barrier Colonic bead expulsion test Chinese hamster ovary Central nervous system N,N-dimethylformamide Effective dose necessary to elicit a 50% effect Guinea pig ileum bioassay Hot-plate test Concentration necessary to produce a 50% effect Interleukin-2 Inhibition constant Methanol Mixed lymphocyte reaction Mouse vas deferens bioassay Sodium hydride Naltriben Naltrindole Peripheral blood mononuclear cells 14-Phenylpropoxymetopon Paraphenylquinone writhing test Rat vas deferens bioassay Structure–activity relationship Subcutaneous Tail-flick test Tosylmethylisocyanid

Synthesis of 14-Alkoxymorphinan Derivatives and Their Pharmacological Actions

65

1 Introduction Although the pain relieving and mood altering effects of extracts of the opium poppy Papaver Somniferum have been known for thousands of years, it was only in the twentieth century that there were substantial advances made in understanding the pharmacology of the active constituents of opium, like morphine [1]. Exciting discoveries such as the identification of the endogenous opioid peptides (enkephalins, endorphins, and dynorphins), the existence of specific opioid receptors and cloning of three opioid receptors (m, d, and k) as members of the G-protein coupled receptor (GPCR) family markedly influenced the course of opioid research over the years [2]. Gene cloning and characterization was an important step to evolve the comprehension of opioid receptors, as proteins that operate in the central and peripheral nervous systems (CNS and PNS), and peripheral tissues [3–6] and control nociceptive, hedonic, emotional, autonomic, neuroendocrine, and immune responses [7, 8]. The driving force behind the many years of synthetical efforts in the opioid field has been the search for an alternative to morphine, which would produce powerful analgesia and would be free of undesirable side effects. All the gained knowledge led to viable strategies in exploring the pharmacotherapeutic potential of the opioid system through the discovery of opioid drugs with alternative therapeutic properties. A diversity of opioid ligands was made available through chemical syntheses, which are appraised as valuable research tools or potential therapeutic agents. Such developments resulted in many pharmacologically interesting and clinically useful opioid compounds possessing different degrees of selectivity for each opioid receptor type, agonist or antagonist activity, and central or peripheral site of action [9–19]. Among opioids, morphinans (Fig. 1) play an important role as therapeutically valuable drugs. Representative examples of the morphinan class of compounds (Fig. 2) are m-opioid analgesic agents for the treatment of moderate-to-severe pain such as naturally occurring alkaloids (e.g. morphine, codeine), semisynthetic derivatives (e.g. oxycodone, oxymorphone, buprenorphine), and synthetic analogs (e.g. levorphanol, butorphanol) [19–21]. Codeine is also an effective antitussive drug. The oxymorphone derivatives naloxone [22] and naltrexone [23] represent 17 H 16 N 10 9 H 8 1 11 B 15 14 2 A 13 C 7 12 3 4 5 6 (-)-Morphinan

Fig. 1 Morphinan

Morphinans: B/C ring fusion cis Isomorphinans: B/C ring fusion trans

66


Fig. 2 Examples of opioid morphinans

two opioid antagonists commonly used as rescue medication to reverse severe side effects (respiratory depression, overdose) induced by opioid agonists [18, 19]. They are also used clinically to treat dependence induced by opioid use and alcoholism [18, 19, 24, 25]. The quaternary derivative of naltrexone, methylnaltrexone [26], was recently introduced for clinical use in the therapy of opioid-induced bowel dysfunction [27]. The k-opioid receptor agonist nalfurafine (AC-820, TRK-820) [28] (Fig. 2) showed efficacy as an antipruritic agent in uremic pruritus patients [29]. Other potential therapeutic indications of opioid morphinans include addiction, depression, feeding behavior, and Parkinson-induced tardive dyskinesia [12, 16, 18, 30–32]. Also, important research tools to investigate opioid receptor multiplicity and function in vitro and in vivo have come from the development of morphinans, including opioid antagonists cyprodime (m) [33], naltrindole (d) [34], and norbinaltorphimine (k) [35] (Fig. 2). During the past decade, 14-alkoxymorphinans have emerged as highly interesting and promising substitution patterns to improve potency and selectivity towards single opioid receptor types affording opioid ligands that besides their scientific value as pharmacological tools, may also have the potential of emerging as novel therapeutic agents. This chapter will cover 14-alkoxymorphinan derivatives including recent chemical developments, pharmacology and ligand-based structure–activity relationships (SAR), with a focus on a number of key findings and selected studies.


67

2 Synthesis 2.1 2.1.1

Synthesis of 14-Alkoxymorphinan-6-ones Introduction of a 14-Alkoxy Group

Introduction of a 14-Alkoxy Group into 7,8-Didehydromorphinans Introduction of a 14-alkoxy group into morphinans was first accomplished by methanolysis of 14-bromocodeinone dimethyl acetale (1) to yield 14-methoxycodeinone (2) (Scheme 1) [36]. An improved synthesis of 14-methoxycodeinone (2) was performed by 14-O-methylation of 14-hydroxycodeinone (3) using either methyliodide [37] or dimethyl sulfate [38] as the alkylating agent in DMF in the presence of NaH (Scheme 2). 14-Ethoxycodeinone (4) was synthesized similarly by alkylation of 2 with diethyl sulfate [38]. Catalytic hydrogenation of 2 and 4 gave the dihydrocodeinones 5 and 6 [38]. Analogously compounds 10 and 11 have been prepared from 7,8-didehydromorphinan 7 via 8 and 9 (using methyliodide or ethyliodide and NaH) (Scheme 3) [39, 40]. Introduction of a 14-Alkoxy Group into Morphinan-6-ones Without 7,8 Double Bond 14-O-Alkylation can also be achieved using oxymorphone-derived morphinan6-ones without a 7,8 double bond employing dimethyl or diethyl sulfate under CH3

CH3

N

N Br

OCH3

14

O

H3CO

1

OCH3 OCH3

H3CO

O

O

2

Scheme 1 CH3

CH3

CH3

N

N

N OH

OR

OR

14

H3CO

O 3

Scheme 2

O

H3CO

O 2 R = Me 4 R = Et

O

H3CO

O 5 R = Me 6 R = Et

O

68

H. Schmidhammer and M. Spetea CH3

CH3

N

CH3

N OH

N OR

OR

14

O

H3CO

O

H3CO

H3CO

7

O 10 R = Me 11 R = Et

8 R = Me 9 R = Et

Scheme 3

CH3

CH3

N

CH3 N

N OH

OR2

OR2

14

O

R

O

12 R = H 13 R = OCH2Ph

R1

O

OR2

R1

O

O

14 R1 = H, R2 = Me

17 R1 = H, R2 = Me

15 R1 = OCH2Ph, R2 = Me 16 R1 = OCH2Ph, R2 = Et

19 R1 = OCH2Ph, R2 = Et

18 R1 = OCH2Ph, R2 = Me

Scheme 4

similar conditions as described above. Thus, 3-deoxygenated oxymorphone (12) and the 3-O-benzyl-substituted oxymorphone derivative 13 yielded 14-alkoxysubstituted enol ethers 14–16, which were hydrolyzed under mild acidic conditions to give ketones 17–19 (Scheme 4) [41, 42]. Introduction of a 14-Alkoxy Group Using Allylic Halides Because of the low reactivity of the tertiary alcohol, alkylation of the C14hydroxyl with alkyl halides such as propyl or isoamyl halides was unsuccessful [Schmidhammer H, unpublished observations]. Therefore, allylic halides were employed to introduce 14-O-alkenyl substituents using similar conditions as described above [43–46]. Catalytic hydrogenation afforded the corresponding 14-Oalkyl derivatives [43–45]. Thus, 14-hydroxy-5-methylcodeinone (20) was treated with 3,3-dimethylallyl bromide in DMF in the presence of NaH to give compound 21, which underwent catalytic hydrogenation to yield 14-O-isoamyl-substituted morphinan 24 (Scheme 5) [43]. Similarly 14-phenylpropoxymorphinans 25 and 26 were prepared from 14-hydroxycodeinone (3) and 21, respectively, via intermediates 22 and 23, which were obtained by alkenylation using cinnamyl bromide (Scheme 5) [44, 45].

Synthesis of 14-Alkoxymorphinan Derivatives and Their Pharmacological Actions CH3

CH3

CH3

N

N

N OH

69

OR1

OR1

14

O

H3CO

R O

20 R = Me 3R=H

H3CO

O

H3CO

R2 O

21 R1 = CH2CH=C(CH3)2, R2 = Me 22 R1 = CH2CH=CHPh, R2 = H 23 R1 = CH2CH=CHPh, R2 = Me

O

R2 O

24 R1 = (CH2)2CH(CH3)2, R2 = Me 25 R1 = (CH2)3Ph, R2 = H 26 R1 = (CH2)3Ph, R2 = Me

Scheme 5

CH3

CH3

CH3

N

N

N OH

OR1

OR1

14

R2O

O

R1 O

3 R1 = H, R2 = Me 27 R1 = Me, R2 = H

R3O

O

R2 O

R3O

O

R2 O

28 R1 = CH2Ph, R2 = H, R3 = Me

30 R1 = CH2Ph , R2 = H, R3 = Me

29 R1 = R3 = CH2Ph, R2 = Me

31 R1 = CH2Ph, R2 = Me, R3 = H

Scheme 6

Introduction of 14-Arylmethoxy Substituents into 7,8-Didehydromorphinans Introduction of 14-arylmethoxy substituents can be achieved starting from 14-hydroxycodeinones (e.g. 3) or 14-hydroxymorphinones (e.g. 27) using arylmethyl bromides (e.g. benzyl bromide) in DMF in the presence of NaH to yield compounds 28 and 29, respectively (Scheme 6). Catalytic hydrogenation leads to the desired N-methylmorphinan-6-ones 30 and 31, respectively [47].

Introduction of 14-Benzyloxy Substituents into Morphinan-6-ones Without 7,8 Double Bond 14-O-Benzylation can also be achieved using oxymorphone and derivatives thereof (e.g. naltrexone). Since 14-O-alkylation of morphinan-6-ones without 7,8 double bond does not proceed as smoothly when other alkylating reagents than dimethyl or diethyl sulfate (see above) are used, the 6-keto function was protected by ketalization [47, 48]. For instance, 3-O-benzyl-protected ketals 32 and 33 were 14-Oalkylated with different benzyl bromides in DMF in the presence of NaH to afford compounds 34 and 35, respectively. Removal of the protecting groups

70

H. Schmidhammer and M. Spetea R

R1

R1

N

N

N OH

OR2

OR2

14

PhH2CO

O

O O

32 R = Me 33 R = CPM

PhH2CO

O

O

O

HO

O

O

34 R1 = Me, R2 = CH2Ph-2,6-diCl

36 R1 = Me, R2 = CH2Ph-2,6-diCl

35 R1 = CPM, R2 = CH2Ph-3-Cl

37 R1 = CPM, R2 = CH2Ph-3-Cl

CPM = cyclopropylmethyl

Scheme 7

R1

R1

N

N OR2

HO

O

R3 O

R1 = Me, allyl, CPM, CBM etc. R2 = Me, Et, Pr, benzyl etc. R3 = H, Me

OR2

HO

O

R3

X

Indolomorphinans: X = NH Benzofuromorphinans: X = O

CPM = cyclopropylmethyl CBM = cyclobutylmethyl

Scheme 8

was accomplished with methanol/HCl to give ketones 36 and 37, respectively (Scheme 7) [47, 48].

2.2

2.2.1

Synthesis of 14-Alkoxy-Substituted Indolo- and Benzofuromorphinans Synthesis from 14-Alkoxymorphinan-6-ones

Indolomorphinans can be prepared from their 6-ketomorphinan precursors via Fischer indole synthesis with phenylhydrazine hydrochloride in glacial acetic acid [43, 49, 50], while benzofuromorphinans can be synthesized from the corresponding 6-ketomorphinans using O-phenylhydroxylamine hydrochloride and methanesulfonic acid in MeOH (Scheme 8) [43].


N

N OH

RO

O

N OR

X

MeOC2OH

NTB: R = H, X = O NTI: R = H, X = NH 38 R = CH2OMe, X = O 39 R = CH2OMe, X = NCH2OMe

O

71

OR

X

HO

X

O

Series A: X = O Series B: X = NCH2OMe

Series C: X = O Series D: X = NH

R = alkyl, arylalkyl

Scheme 9

2.2.2

Synthesis Starting from Naltrindole or Naltriben

Naltrindole (NTI) [34, 51] and naltriben (NTB) [52] are prototype d-opioid receptor antagonists belonging to the indolo- and benzofuromorphinan series of compounds. Starting either from NTI or NTB, a more efficient synthesis of 14-alkoxy-substituted indolo- and benzofuromorphinans in three steps was reported, whereby the 14-O-alkyl group is introduced in the penultimate step of the procedure [53]. Thus, protection of the C3-hydroxyl group of NTB and of both the C3-hydroxyl group and indole nitrogen of NTI with methoxymethyl bromide gave methoxymethylprotected derivatives 38 and 39, respectively. Subsequent 14-O-alkylation with different alkyl bromides in DMF using NaH as the base afforded 14-O-alkylated series A and series B, respectively. Acid hydrolysis (MeOH/1 N HCl) yielded the desired benzofuromorphinans (series C) and indolomorphinans (series D) (Scheme 9). Similarly benzofuromorphinans of series C were prepared using the triisopropylsilyl protecting group instead of methoxymethyl [53].

2.3

Introduction of New Functionalities to 14-Alkoxymorphinan-6-ones

Among the most indispensable analgesics, morphinan-6-ones such as oxymorphone, oxycodone, and dihydromorphinone have their main importance in clinical use. As earlier shown, hydrazone, oxime, carbazone, and semicarbazone derivatives of these morphinan-6-ones exhibit high affinity at the m-opioid receptor [54–56] and exhibit high antinociceptive potency in addition to less pronounced side effects [19, 57–60]. Therefore, it remains a promising synthetic task to convert the carbonyl group into various functionalities. 2.3.1

Incorporation of Acrylonitrile Substructures into Morphinans

An interesting approach to incorporate acrylonitrile substructures into morphinans is the van Leusen homologation reaction in which the standard reagent

72


tosylmethylisocyanid (TosMIC) reacts with carbonyl compounds to give the corresponding nitriles with one additional carbon atom [61].

Van Leusen Reaction Applied to Morphinan-6-ones Without 7,8 Double Bond Applying the van Leusen reaction [61] to oxycodone (40) and 14-O-methyloxycodone (41), the ether bridge between positions 4 and 5 was opened and the respective acrylonitrile derivatives 42 and 43 were formed (Scheme 10) [62].

Van Leusen Reaction Applied to 7,8-Didehydromorphinan-6-ones In contrast to the cyanation of oxycodone (40) and 14-O-methyoxycodone (41) with TosMIC (Sect. 2.3.1.1), 7,8-didehydromorphinan-6-ones such as 14-hydroxycodeinone (3), 14-methoxycodeinone (2), and 14-cinnamyloxycodeinone (44) were found to yield 6,7-didehydrocarbonitriles 45–47 with retainment of the 4,5-epoxy ring under these conditions (Scheme 11) [63].

CH3

CH3

N

N OR

H3CO

O

O

OR

H3CO

40 R = H 41 R = Me

OH

CN

42 R = H 43 R = Me

Scheme 10

CH3

CH3

N

N OR

H3CO

O

O

3R=H 2 R = Me 44 R = cinnamyl

Scheme 11

OR

H3CO

O

CN

45 R = H 46 R = Me 47 R = cinnamyl


2.3.2

73

Synthesis of 6-Amino Acid-Substituted 14-Alkoxymorphinans

Morphinans with zwitterionic moieties show greatly reduced access to the CNS without substantially decreased opioid receptor activity [64, 65]. A series of 6-amino acid conjugates (glycine, alanine, and phenylalanine) of the highly potent opioid analgesic 14-O-methyloxymorphone (48) [41] has been synthesized in an effort to obtain opioid agonists with high potency that would have reduced ability to cross the blood–brain barrier (BBB) [66] (Scheme 12). A novel synthetic approach for the synthesis of 6-amino acid derivatives in the morphinan series was employed. Thus, the tert-butyl ester derivatives 49–54 were prepared from 14-O-methyloxymorphone (48) by reductive amination with the respective tert-butyl ester hydrochlorides and sodium cyanoborohydride in ethanol. After separating the epimers by column chromatography, esters 49–54 were treated with tetrafluoroboric acid to afford compounds 55–60 as bis(tetrafluoroborates).

3 Pharmacological Actions 3.1

14-Alkoxymorphinan-6-ones

Introduction of a 14-methoxy group in oxymorphone resulted in 14-O-methyloxymorphone (48) (Fig. 3) [41] which not only increases affinity to opioid receptors in CH3

CH3

N

N OCH3

HO

O

O

OCH3

HO

O

N

R

H 48

49 R = CH2COOC(CH3)3, H-6b 50 R = CH2COOC(CH3)3, H-6a 51 R = (S)-CH(CH3)COOC(CH3)3, H-6b 52 R = (S)-CH(CH3)COOC(CH3)3, H-6a 53 R = (S)-CH(CH2Ph)COOC(CH3)3, H-6b 54 R = (S)-CH(CH2Ph)COOC(CH3)3, H-6a 55 R = CH2COOH, H-6b 56 R = CH2COOH, H-6a 57 R = (S)-CH(CH3)COOH, H-6b 58 R = (S)-CH(CH3)COOH, H-6a 59 R = (S)-CH(CH2Ph)COOH, H-6b 60 R = (S)-CH(CH2Ph)COOH, H-6a

Scheme 12

74


Fig. 3 Oxymorphone derivatives

CH3 N OR1

HO

O

R2

O

48 R1 = Me, R2 = H 61 R1 = CH2Ph, R2 = H 62 R1 = Me, R2 = Me 63 R1 = CH2Ph, R2 = Me 64 R1 = (CH2)3Ph, R2 = Me

Table 1 Opioid receptor binding affinities of oxymorphone derivatives [45, 47] Ki d (nM) Ki k (nM) Selectivity ratio Compound Ki m (nM) d/m k/m 48 0.10 4.80 10.2 48 102 61 0.12 2.14 1.18a 18 10 62 0.15 13.3 25.2 89 168 63 0.18 3.67 2.46a 20 14 0.7 2 64 0.20 0.14 0.40a Oxymorphone 0.97 80.5 61.6a 83 63 Morphine 6.55 217 113 33 17 a Guinea rat or guinea pig brain membranes were used in binding assays Ki inhibition constant Table 2 Antinociceptive potencies of oxymorphone derivatives [45, 47] HP TF PPQ Compound HP ED50 (mg/kg) ED50 (mg/kg) ED50 (mg/kg) AD50 (nmol/kg) 48 53 – – – 61 9.6 – – – 62 280 30 30 9.0 63 65 64 – 0.10 0.08 0.16 Morphine 6,690 850 1,920 400 AD50 and ED50 analgesic and effective dose, respectively, necessary to elicit a 50% effect in mice after s.c. administration; HP hot-plate test; TF tail-flick test; PPQ paraphenylquinone writhing test; – not tested

in vitro opioid binding assays, while retaining the m-receptor selectivity of oxymorphone (Table 1), but also markedly enhances the antinociceptive potency [41, 47, 67]. The presence of the methoxy group at position 14 resulted in a considerable increase in m-opioid agonist activity in the mouse vas deferens (MVD) bioassay with derivative 48 having over 800-fold increased potency compared to oxymorphone [67]. 14-O-Methyloxymorphone (48) was characterized by potent agonist properties in the guinea pig ileum (GPI) and rat vas deferens (RVD) bioassays with IC50 values of 2 nM and 143 nM, respectively [47, 67]. It was reported to possess about 40-fold higher antinociceptive potency than its parent compound, and was up to 400-fold more potent than the “golden standard” for pain therapy, morphine, in rodent models of acute nociception i.e. hot-plate test in mice (Table 2) [41, 47] and tail-flick test in rats [67, 68] and mice [67], after subcutaneous (s.c.) administration. Compound 48 induces the classical opioid side effects of the conventional m-opioid


75

analgesics including respiratory depression [41], physical dependence [41], and constipation [47] after s.c. administration to mice. Using 14-O-methyloxymorphone (48) as the lead structure, a series of novel 14-alkoxy-substituted (i.e. allyloxy, benzyloxy, naphthylmethoxy) morphinan6-one derivatives was prepared and pharmacologically evaluated [47]. The analog having a benzyl group in position 14 (61, Fig. 3) preserved the high affinity at the m-opioid receptor of the parent compound 48 but had significantly increased interaction with the other opioid receptor types, d and k (Table 1), thus resulting in a reduction in m-receptor selectivity. It was a highly potent m-opioid agonist in the GPI (IC50 ¼ 0.9 nM) and MVD bioassays (IC50 ¼ 1.5 nM), showing 2- and 20-fold, respectively, increased agonist potency in comparison to the 14-O-methyl analog 48 [47]. Antinociceptive potency in the hot-plate test after s.c. administration in mice was further augmented through the introduction of a 14-benzyloxy group compared to the methoxy group, by about 5-fold increase when compared to the parent compound 48, and by about 500-fold increase versus morphine (Table 2). Remarkable was the observation that the 14-benzyloxy substitution gave rise to a m-opioid agonist eliciting negligible constipative activity at the analgesic dose, using the colonic bead expulsion test (CBE) in mice after s.c. administration. Analog 61 was 7-fold less constipating than 14-O-methyloxymorphone (48) as assessed by the CBE (ED50 CBE/AD50 hot-plate test ¼ 2.8 and 0.4, respectively [47]). A second approach to obtaining m-opioid analgesics in the N-methylmorphinan6-one class exhibiting more favorable pharmacological features was based on the introduction of a methyl group in position 5 of 14-O-methyloxymorphone, leading to 14-methoxymetopon (62) (Fig. 3). Following its first description in 1990 [69], a number of reports on in vitro and in vivo activities of 14-methoxymetopon (62) were published in the past years [47, 67, 70–78]. The presence of the 5-methyl substituent had no major effect on affinity to the m-opioid receptor compared to 14-O-methyloxymorphone (48), while a slight decrease in binding to d- and k-receptors was observed, thus, maintaining the m-receptor selectivity (Table 1). The 5-methyl-substituted derivative 62 showed high agonist activity and comparable potency to the 5-unsubstituted derivative 48 in the GPI, MVD, and RVD bioassays [47, 67]. The high agonist potency of compound 62 was also established using [35S]GTPgS functional assay in rat brain [74], calf striatum [77], and Chinese hamster ovary (CHO) cells expressing mouse m-receptor splice variants [77]. A tritium-labeled form of 14-methoxymetopon, [3H]14-methoxymetopon, has also been prepared [74] and possesses high affinity and selectivity for native and recombinant m-opioid receptors [74, 77]. In vivo, introduction of a methyl group in position 5 in 14-O-methyloxymorphone (48) resulted in somewhat of a decrease in antinociceptive potency, i.e. 5-fold in the hot-plate test (Table 2) and 2-fold in the tail-flick test in mice after s.c. administration for compound 62 [67]. Pharmacological studies consistently reported on the high antinociceptive activity of the m-opioid agonist 62 ranging between 24- and 20,000-fold in comparison to morphine in diverse models of acute nociception (i.e. hot-plate test [44, 47, 71], tail-flick test [44, 67, 73, 76, 79] and tail electrical stimulation test [72]), and visceral pain (acetic acid- [69, 79], and

76


paraphenylquinone (PPQ)-induced writhing test [44]) in rodents (mice, rats) after systemic administration. It was equipotent to sufentanil in eliciting analgesia in the skin-twitch test in canines after intravenous administration [70]. Antinociceptive effects of compound 62 after s.c. administration were also reported in inflammatory pain using a rat model of carrageenan-induced hindpaw inflammation [75]. The m-agonist 62 at a dose of 20 mg/kg showed comparable efficacy to morphine (2 mg/kg) in inhibiting nociceptive behavior in inflamed hindpaws, thus being 100-fold more potent than morphine in experimental inflammatory pain. Despite its high analgesic potency, 14-methoxymetopon (62) was reported to display little respiratory depression, hypotension, bradycardia, or sedation compared to sufentanil in canines [70]. It decreases gastrointestinal transit far less than morphine and reportedly develops lower levels of tolerance [76, 79], physical dependence [79], and a diminished propensity to cause convulsions in mice [79]. Behavioral studies showed that compound 62 is far more effective than morphine in reducing the emotive/affective component of pain and in producing an anxiolytic effect in rats [72]. Therefore, it was depicted as a highly potent m-opioid analgesic with a pharmacological profile distinct from that of traditional m-opioid agonists such as morphine. Physicochemical properties are often an important consideration for understanding the behavior of bioactive molecules and their correlation with pharmacological activities. Experimentally determined liphophilicity of oxymorphone derivatives, 14-O-methyloxymorphone (48) and 14-methoxymetopon (62), have been recently reported [67]. 14-O-Methyloxymorphone (48) displays comparable lipophilicity to its parent compound oxymorphone (logP values of 0.60 and 0.67, respectively), but shows greater m-opioid receptor affinity (Table 1) and behaves as a more potent agonist in vitro and in vivo [67]. Introduction of a methyl group in position 5 in 14-O-methyloxymorphone (48) leads to a more liphophilic molecule 62 (logP ¼ 1.12), while showing comparable antinociceptive potency in vivo and similar activities i.e. m-opioid affinity and agonist potency in vitro. Further chemical derivatization having 14-methoxymetopon (62) as the lead, resulted in a series of 14-arylalkyloxymetopon derivatives i.e. benzyloxymetopon [47] and phenylpropoxymetopon [45]. Replacement of the 14-methoxy substituent by benzyloxy or phenylpropoxy gave rise to analogs 63 and 64 (Fig. 3) displaying markedly enhanced binding affinities to both d- and k-opioid receptors, while the high affinity at the m-receptor remains unchanged. This resulted in a decrease in selectivity for the m-receptor of derivative 63 and a complete loss of m-selectivity for 14-phenylpropoxymetopon (PPOM, 64) (Table 1). Both analogs were potent antinociceptive agents in mice after s.c. administration (Table 2). The most striking finding was that compound 64 acted as an extremely potent opioid agonist in vivo, showing considerable increased antinociceptive potency (60- to 400-fold) compared to 14-methoxymetopon (62) in different nociceptive tests (i.e. hot-plate, tailflick, and PPQ writhing tests) in mice after s.c. administration (Table 2). PPOM (64) was 8,500- and 24,000-fold more active in the hot-plate and tail-flick tests, respectively, than morphine. It was reported that the 14-phenylpropoxy-substituted analog 64 is even more potent as an antinociceptive agent than etorphine, a well-known


77

opioid morphinan used in veterinary medicine [80, 81]. The 3-O-methyl ether of PPOM (26) showed unexpectedly high antinociceptive potency after s.c. administration in mice although its potency was about 10- to 55-fold lower than that of PPOM (64) [45]. On the basis of SAR studies, it was evident that the nature of the substituent at position 14 in N-methylmorphinan-6-ones has a major impact on their abilities to interact with opioid receptors leading to profound alteration in the pharmacological profile in this class of opioid compounds. A 14-phenylpropoxy substituent would also increase liphophilicity of 14-methoxymetopon (62) which might broaden the therapeutic scope, for example use in transdermal systems. Established and generally accepted SAR models have assigned critical importance in defining the pharmacological profile of agonist/antagonist of morphinan6-ones to the substituent at the morphinan nitrogen [82, 83]. Substituents such as cyclopropylmethyl or allyl at the nitrogen have been commonly associated with an antagonist character. Well-known examples are the nonselective opioid receptor antagonists, naloxone and naltrexone (Fig. 2). The introduction of a 14-O-methyl or ethyl group into naloxone and naltrexone, respectively, did not significantly alter the in vitro and in vivo potencies of these two antagonists [12, 38]. A 14-phenylpropoxy substitution in naloxone and naltrexone afforded derivatives 65 and 66, respectively (Fig. 4) [44] having high affinity at all three opioid receptor types, with low Ki values in the subnanomolar range (Table 3). Surprisingly, they acted as opioid agonists in vivo, inducing potent antinociceptive effects in different nociceptive tests (i.e. hot-plate, tail-flick and PPQ-writhing tests) in mice after s.c. administration (Table 3). Both 14-phenlypropxy analogs 65 and 66 were several folds more potent than morphine (100- and 300-fold in the hot-plate and tail-flick tests, respectively for compound 65, and 400-, 600- and 60-fold in the hot-plate, tail-flick and PPQ-writhing tests, respectively, for compound 66). They even R N O(CH2)3Ph

Fig. 4 14-Phenylpropoxysubstituted derivatives of naloxone and naltrexone

65 R = allyl 66 R = CPM CPM = cyclopropylmethyl

HO

O

O

Table 3 In vitro and in vivo opioid activities of 14-phenylpropoxy derivatives of naloxone and naltrexone [44] Compound Receptor binding assay Antinociception Ki m (nM) Ki d (nM) Ki k (nM) Selectivity ratio ED50 (mg/kg s.c.) d/m k/m HP TF PPQ 65 0.20 0.26 0.11 0.55 1.30 0.006 0.006 – 0.48a 0.41a 1.96 1.84 0.002 0.003 0.006 66 0.34a a C6 rat glioma cells or rat brain membranes were used in binding assays Ki inhibition constant; ED50 effective dose necessary to elicit a 50% effect in mice after s.c. administration; HP hot-plate test; TF tail-flick test; PPQ paraphenylquinone writhing test; – inconsistent dose response

78


surpassed 14-methoxymetopon (62) in analgesic potency and were essentially inactive as antagonists in the tail-flick test in mice versus morphine [44]. This observation is in contrast to the findings upon 14-O-methylation and ethylation of naloxone and naltrexone which did not significantly alter binding affinities and agonist potency. These derivatives of naloxone and naltrexone were reported to be essentially devoid of agonist activities in the MVD bioassay and nociceptive assays (i.e. hot-plate and tail-flick tests) in rats and were pure opioid antagonists [12, 38]. Another study [84] reported on the effect of introduction of a 14-phenylpropoxy group in cyprodime and analogs (Fig. 5) on in vitro and in vivo opioid activities. The presence of a 14-phenylpropoxy substituent in cyprodime, a selective m-opioid receptor antagonist (Fig. 2) markedly improved the interaction with the m-receptor, noted as a considerable increase in m-binding affinity (Ki values of 10.6 nM for cyprodime, 0.34 nM for the analog 67, and 0.40 nM for analog 68) and similar or higher m-receptor selectivity compared to cyprodime (Table 4). In the [35S]GTPg functional assay, both derivatives 67 and 68 were low efficacy partial agonists at mand d-opioid receptors while at the k-receptor, antagonist activity was shown by the 14-phenylpropoxy analog 67 and some partial activity by the derivative 68. Compound 67 was tested for antagonism at human k-receptors and showed a Ke value of 2.52 nM in the [35S]GTPgS functional assay [84]. In vivo, it was found that cyprodime was converted into potent antinociceptive agents through the introduction of a 14-phenylpropoxy group (Table 4). When tested in mice after s.c. administration using hot-plate, tail-flick and PPQ-induced writhing tests, derivatives 67 and 68 were considerably more potent than morphine, with phenol 68 showing the highest antinociceptive potency (21-, 38- and 300-fold in the hot-plate, tail-flick and PPQ-writhing tests, respectively). It also showed similar potency to 14-methoxymetopon (62) in all three nociceptive assays. Both 14-phenylpropoxy-

N O(CH2)3Ph

Fig. 5 14-Phenylpropoxysubstituted derivatives of cyprodime

OR

67 R = Me 68 R = H

O

Table 4 In vitro and in vivo opioid activities of 14-phenylpropoxy derivatives of cyprodime [84] Compound Receptor binding assay Antinociception Selectivity ratio ED50 (mg/kg s.c.) Ki m (nM) Ki d (nM) Ki k (nM) d/m k/m HP TF PPQ 67 0.34 16.9 7.36 55 20 0.30 0.28 0.06 68 0.40 5.06 5.84 13 15 0.04 0.05 0.0014 Cyprodime 10.6 414 109 39 10 – – – Rat brain membranes were used in binding assays Ki inhibition constant; ED50 effective dose necessary to elicit a 50% effect in mice after s.c. administration; HP hot-plate test; TF tail-flick test; PPQ paraphenylquinone writhing test; – not applicable


79

substituted derivatives of cyprodime where inactive as antagonists against morphine in the tail-flick test in mice [84]. The SAR studies and pharmacological findings reported in the two studies [44, 84] provide evidence that not necessarily the nature of the substituent at the nitrogen in morphine-like compounds but rather residues occupying a defined position in the vicinity to the morphinan nitrogen seem to be responsible for the agonist/antagonist action. This observation seems to revise the traditionally established SAR models [82, 83]. In the series of morphinan-6-ones with a 14-phenylpropoxy group, even with N-substituents such as cyclopropylmethyl and allyl that are usually associated with distinct antagonist properties, agonists and highly potent analgesics were obtained.

3.2

6-Cyanomorphinans

The C6 carbonyl group of morphinan-6-ones can be easily chemically modified, and pharmacological studies have shown that such modifications generally do not affect the opioid character of the ligand [54–56], while the resulting compounds have high antinociceptive potencies, together with reduced unwanted side effects such as respiratory depression and gastrointestinal inhibition [57, 58]. These included for example hydrazones, oximes, and semicarbazone derivatives of N-methyl-6-ketomorphinans. On the basis of the pharmacological findings that 4,5-oxygen bridge-opened morphinan-6-ones (N-methyl substituted [41] and N-cycloproplymethyl substituted [84]) have increased affinities at the m-opioid receptor and higher antinociceptive potency than their 4,5-oxygen-bridged analogs, it was investigated how and to what degree the presence of a 6-cyano group would affect the opioid receptor binding profile and in vivo antinociceptive potency compared to their 6-keto analogs. The report [85] on acrylonitrile incorporated 4,5-oxygen bridge-opened Nmethylmorphinans (Fig. 6) described that a 6-cyano substituent leads to compounds (42, 43, and 69) with high affinity at the m-opioid receptor and decreased interaction with d- and k-receptors, thus being m-selective (Table 5). Also, the replacement of the 6-keto group by a 6-cyano group did not significantly affect m-opioid receptor affinity and retained the low binding to the d- and k-sites. In vivo, the 6-cyanomorphinans (42, 43, and 69) acted as potent antinociceptive agents in three nociceptive CH3

CH3 N OR1

H3CO

OR2

CN

N

42 R1 = H, R2 = H 43 R1 = Me, R2 = H 69 R1 = H, R2 = Me

OR1

70 R1 = H, R2 = H 71 R1 = Me, R2 = H 72 R1 = H, R2 = Me

H3CO

Fig. 6 6-Cyanomorphinans and their 6-keto analogs

OR2

O

80


Table 5 In vitro and in vivo opioid activities of 6-cyanomorphinans and their 6-keto analogs [85] Compound Receptor binding assay Antinociception ED50 (mg/kg s.c.) Ki m (nM) Ki d (nM) Ki k (nM) Selectivity ratio d/m k/m HP TF PPQ 42 31.7 498 1648 16 52 0.50 1.88 0.18 43 5.38 197 378 37 70 0.25 0.21 0.11 69 2.44 107 364 44 149 0.15 0.018 0.026 70 32.6 881 763 27 23 2.60 – – 71 4.65 180 592 39 127 0.29 – – 72 3.88 91.6 693 24 179 0.14 – – Oxycodone (40) 43.6 1087 2658 25 61 1.37 0.94 0.38 Rat brain membranes were used in binding assays Ki inhibition constant; ED50 effective dose necessary to elicit a 50% effect in mice after s.c. administration; HP hot-plate test; TF tail-flick test; PPQ paraphenylquinone writhing test; – not tested

tests (i.e. hot-plate, tail-flick and PPQ-writhing tests) in mice after s.c. administration. They were either more active or equipotent to their 6-keto counterparts (70–72), and also to oxycodone (40) (Table 5). The presence of a 14-methoxy substituent (compound 43) instead of a hydroxy group in 6-cyanomorphinan 42 not only increases in vitro binding affinity at the m-receptor, but also enhances the antinociceptive potency (Table 5), a similar observation being made for the morphinan-6-ones [44, 47, 84]. The chemically highly versatile acrylonitrile substructure allows for further conversion into various and more polar derivatives, and, therefore bears the potential to open up a new field in morphinan chemistry and opioid pharmacology.

3.3

6-Amino Acid-Substituted 14-Alkoxymorphinans

The important function of opioid receptors outside of the CNS in different pathophysiological conditions has gained considerable attention during the past decades, and nowadays opioid researchers and physicians are beginning to appreciate the new “peripheral” paths for improved and innovative therapy [6, 8, 86]. Medicinal chemistry and opioid pharmacology focuses increasingly on exploring the potential of peripheral opioid receptors through the development of peripherally acting opioid compounds as therapeutic agents in the treatment of disease states such as pain, inflammatory diseases, and gastrointestinal disorders [14, 18, 27, 87–89]. Approaches to limit access to the CNS and as a consequence also to reduce the occurrence of unwanted CNS side effects include chemical modifications at opioid compounds that increase their hydrophilicity such as incorporation of highly polar substituents. Earlier works to develop peripherally selective opioids have primarily focused on the quaternization of the nitrogen in known opioid morphinans such as morphine, oxymorphone, or naloxone [14, 26, 90]. Although quaternary compounds show reduced ability to cross the BBB relative to their tertiary amine


81

precursors, they have considerably lower affinity to opioid receptors [26, 90]. Quaternization also trends to reduce potency in vivo. In order to avoid these limitations, opioids with hydrophilic groups attached at the C6 position of the morphinan skeleton have been synthesized from b-oxymorphamine [64], b-naltrexamine [64], and b-funaltrexamine [91]. Such opioids with zwitterionic moieties showed reduced penetration to the CNS, without substantially decreased opioid receptor activity [64, 65]. The study [66] reported on the incorporation of an amino acid residue into the highly potent centrally acting m-opioid analgesic 14-O-methyloxymorphone leading to zwitterionic 6-amino acid-substituted derivatives (glycine, alanine, and phenylalanine) (55–60) of 14-O-methyloxymorphone (48) (Scheme 12) as potent m-opioid receptor agonists with limited access to the CNS. In binding studies with rat brain membranes [92], all 6-amino acid conjugates 55–60 displayed high affinities (Ki ¼ 0.77–2.58 nM) at the m-opioid receptor (Table 6). They were potent m-opioid agonists in the MVD bioassay (IC50 ¼ 5.52–26.8 nM). While the a-amino acid epimers were favored by m-opioid receptors, the b-epimers have increased interaction with d-binding sites (Table 6). Only the 6b-phenylalanine 60 conjugate showed some preference for d- over m-opioid receptors, and d-opioid receptor agonist activity in the MVD bioassay (Table 6). The m- and d-receptor affinities remained somewhat similar to the parent compound 14-O-methyloxymorphone (48), whereas affinities at the k-receptor were significantly decreased independent of a- or b-substitution (Table 6). A number of in vivo pharmacological studies reported that amino acid substitution in position 6 of 14-O-methyloxymorphone (48) afforded derivatives 55–60 that produce potent antinociceptive actions via peripheral mechanisms after s.c. administration in different pain models such as acute nociception i.e. tail-flick test [68], inflammatory pain i.e. formalin test [68] and carrageenan-induced hindpaw inflammation [75] and visceral pain i.e. acetic acid-induced writhing test [93]. In the tail-flick test in the rat, the 6-amino acid derivatives were 19- to 209-fold more potent than morphine (Table 7) and showed similar potency to fentanyl after s.c Table 6 Opioid receptor binding affinities and agonist potencies of 6-amino acid-substituted derivatives of 14-O-methyloxymorphone (48) [92] Compound Receptor binding assay MVD bioassay Ki m (nM) Ki d (nM) Ki k (nM) Selectivity ratio IC50 (nM) d/m k/m 55 0.89 15.4 43.2 17 49 26.8 56 0.83 7.86 44.8 9.5 54 7.00 57 0.77 26.9 142 35 184 12.2 58 1.90 7.71 63.7 4.1 34 19.6 59 0.95 3.67 28.5 3.9 30 5.52 60 2.58 1.03 151 0.40 59 6.07 48 0.10 4.80 10.2 48 102 7.76 Rat brain membranes were used in binding assays Ki inhibition constant; MVD mouse vas deferens; IC50 agonist concentration necessary to produce a 50% effect

82


Table 7 Antinociceptive potencies of 6-amino acidsubstituted derivatives of 14-O-methyloxymorphone (48) [68]

ED50 (nmol/kg s.c.) Tail-flick test Formalin test I phase II phase 55 58.5 72 110 56 29.0 125 204 57 68.9 – – 58 53.4 – – 59 315 171 292 60 >3,600 79 107 48 14.9 – – Morphine 6,053 1,613 1,472 Fentanyl 38.6 – – ED50 effective dose necessary to elicit a 50% effect in rats after s.c. administration; – not tested Compound

administration [68]. Both s.c. (Table 7) and local intraplantar administration [94] of the zwitterionic compounds 55, 56, 59, and 60 produced antihyperalgesic effects in the formalin test in the rat showing greater potencies than morphine. Potent and significant antinociceptive behavior was also reported after s.c. administration of the 6b-glycine conjugate 56, using a rat model of prolonged inflammatory pain, i.e. unilateral carrageenan-induced hindpaw inflammation [75] and a mouse model of abdominal visceral pain [93]. Antihyperalgesic and antiallodynic effects of compounds 55, 56, 59, and 60 were also described in rats with neuropathic pain after local intraplantar administration [94]. Generally, antinociceptive actions of the 6-amino acid-substituted derivatives of 14-O-methyloxymorphone 55–60 were considerably longer-lasting (2–3 h in the tail-flick and writhing tests and up to 4 h in carrageenan-induced inflammatory pain) compared to the clinically relevant m-opioid analgesics, morphine and fentanyl, and the structurally related and centrally acting m-opioids, 14-O-methyloxymorphone (48) and 14-methoxymetopon (62) (30–60 min) [68, 75, 93]. Also, a significant and long-lasting antinociceptive effect (up to 4 h) was reported after oral administration of the 6b-glycine conjugate 56 (10 mg/kg) to rats with carrageenan-induced inflammatory pain [75]. Opioid analgesics with limited access to the CNS that could be administered systemically or orally are of high relevance for clinical use.

3.4

14-Alkoxy-Substituted Indolo- and Benzofuromorphinans

The d-opioid receptor was the first opioid receptor type to be cloned in 1992 [2], and its molecular and functional characterization was largely based on the use of the d-selective indolomorphinan NTI (Fig. 2) and its benzofuran analog naltriben (NTB). SAR studies on series of NTI and NTB analogs focusing on the introduction of alkoxy groups at position 14 were reported [43, 50] (Fig. 7). Further chemical modifications, SAR and biological investigations on 14-alkoxy analogs of NTI and

Synthesis of 14-Alkoxymorphinan Derivatives and Their Pharmacological Actions 73 R1 = CPM, R2 = Me, R3 = H, 74 R1 = CPM, R2 = Et, R1 N

83

X = NH

R3 = H,

X = NH

75 R1 = allyl, R2 = Me, R3 = H,

X = NH

76 R1 = allyl, R2 = Et, R3 = H,

X = NH

77 R1 = CPM, R2 = Me, R3 = Me, X = NH

OR2

78 R1 = CPM, R2 = Et,

R3 = Me, X = NH

79 R1 = CPM, R2 = Pr, R3 = Me, X = NH HO

O

R3

X

80 R1 = allyl, R2 = Me, R3 = H,

X = N-Me

81 R1 = allyl, R2 = allyl, R3 = H,

X = N-allyl

82 R1 = CPM, R2 = Et,

R3 = H,

X=O

83 R1 = allyl, R2 = Et, R3 = H,

X=O

CPM = cyclopropylmethyl

Fig. 7 14-Alkoxy-substituted indolo- and benzofuromorphinans Table 8 Opioid receptor binding affinities of 14-alkoxy-substituted indolo- and benzofuromorphinans [43, 50, 95] Compound Ki d (nM) Ki m (nM) Ki k (nM) Selectivity ratio m/d k/d 73 1.44 68.7 103 48 72 74 0.40 97.0 313 242 783 75 3.8 869 233 229 134 76 2.6 1368 349 526 134 77 1.15 284 421 247 366 78 0.78 340 134 436 172 79 5.3 555 504 105 95 80 1.97 731 149 371 76 81 7.3 1001 763 137 105 82 1.18 185 94 157 80 83 1.20 22.8 68 19 57 NTI 0.14 13.0 15.8 93 113 NTB 0.15 27.1 23.4 180 156 Rat brain membranes were used in m- and d-receptor binding assays and guinea pig brain membranes were used in the k-receptor binding assay Ki inhibition constant

NTB differently substituted in position 5 and 17 and at the indole nitrogen were described [43, 50, 95–98]. The introduction of a 14-alkoxy instead of the 14-hydroxy group present in NTI resulted in somewhat lower binding affinity at the d-receptor, while d-receptor selectivity was considerably increased (compounds 74–81) (Table 8). The nature of the substituent in position 14 was reported to be an important determinant for the interaction with d-receptors. A 14-ethoxy group in indolomorphinans seems to be superior to 14-methoxy and 14-propoxy substitution concerning d-affinity and selectivity. The presence of a 14-ethoxy group (compounds 74, 76, and 78) resulted in higher affinity and selectivity for the d-receptor compared to their 14-methoxysubstituted derivatives 73, 75, and 77, respectively, and 14-propoxy analog 79

84


(Table 8). Arylalkoxy substituents in position 14 reduced d-affinity and selectivity [50]. The introduction of a 5-methyl group in the 14-methoxy indolomorphinan 73 did not significantly change d-receptor affinity, but decreased affinities at m- and k-receptors, resulting in higher m/d and k/d selectivity ratios (247 and 366, respectively) for compound 77. Substitution at the indole nitrogen with methyl (compound 80) or allyl groups (compounds 81) does not have much influence on d-receptor affinity and selectivity when compared to the unsubstituted analogs (Table 8). The nature of X (O-, NH-, or N-substituted) did not largely affect d-receptor affinity and selectivity. Also, replacement of the 17-cyclopropylmethyl group by an allyl substituent has little influence on binding at the d-receptor [43, 50]. The higher d-receptor selectivity of the 14-ethoxy-substituted indolomorphinans 76 and 78 compared to NTI based on opioid receptor binding assays was also reported in [35S]GTPgS functional assays [43, 95]. Potent antagonist activities at the d-receptor were described for other 14-alkoxy-substituted indolo- and benzofuromorphinans compared to NTI and NTB [43, 50, 97]. The interesting opioid receptor activity profile of derivative 78 makes this compound a useful tool to selectively antagonize d-opioid receptor agonist-induced effects or to assess receptor specificity of opioid ligands in vitro and in vivo [74, 99–101]. Highly interesting was the observation reported in several studies [102–104] that the indolomorphinan d-opioid receptor antagonist NTI significantly prolongs renal allograft survival in the rat and inhibits allogenic mixed lymphocyte reaction (MLR) in vitro, similar to the clinically used immunosuppressant cyclosporin A. This remarkable finding brought attention to this class of d-opioid antagonists as potential immunosuppressive agents in organ transplantation and inflammatory diseases. The 14-ethoxy-5-methyl-substituted derivative of NTI (78), which showed higher selectivity and antagonist potency towards the d-opioid receptor than NTI, was reported to inhibit rat lymphocyte proliferation in vitro (IC50 ¼ 0.54 mM), being more active than NTI (IC50 ¼ 6.93 mM) [95]. Other structurally related selective d-opioid antagonists 73, 75, 77, and 83 were also found to possess immunosuppressive activity in vitro in human peripheral blood mononuclear cells (PBMC) at a concentration of 10 mM [96]. Similar to NTI, they reduce interleukin2 (IL-2) production in mouse and human lymphocytes [96]. The immunosuppressive action of NTI and close derivatives was reported not to be mediated through the d-opioid receptor or any of the other opioid receptor types (m and k) using in vitro MLR with splenocytes from d and triple m/d/k opioid receptor knockout mice [105]. These observations have shed new light on the immunoregulatory mode of action of this class of opioids. It has been postulated that structural determinants of the indolo moiety in NTI and derivatives 73, 75, 77, and 78 or the corresponding benzofuran group in 83 (Fig. 7) appear to be required for the immunosuppressive activity, since naltrexone was inactive in the MLR suppression [105]. Recently, NTI and 14-alkoxy-substituted indolomorphinan derivatives 77 and 78 have been reported to suppress tryptophan degradation and neopterin formation (markers for diseases that are associated with inflammation or altered immune function [106]), in mitogeninduced human PBMC, with derivatives 77 and 78 exhibiting slightly higher potency than NTI [98]. Thus, there is evidence that such opioid indolomorphinans


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besides their significant scientific value as pharmacological tools might also have potential for therapeutic use in immunopathological disorders.

4 Conclusions In recent years, numerous amounts of synthetical efforts have resulted in the development of new 14-alkoxymorphinans together with a significant expansion of knowledge on their pharmacology and ligand-based structure–activity relationships. The nature of the substituent at position 14 in this class of compounds has a major impact on the ability of morphinans to interact with opioid receptors, leading to qualitative and quantitative differences in biological and pharmacological activities. Targeting position 14 represents a viable approach for tuning the pharmacological properties of this class of opioids. Additional substitution patterns in positions 5, 6, and 7 in 14-alkoxymorphinans results in compounds with distinct opioid activities and provided further understanding of structural requirements and functional selectivity. Appropriate molecular manipulations of the morphinan template can afford novel opioid ligands, which besides their scientific value as pharmacological tools, may also have the potential of emerging as novel therapeutic agents for the treatment of human disease states. These advances may lead to strategies in investigating the pharmacotherapeutic potential of the opioid system through the discovery of novel and innovative opioid drugs from the 14-alkoxymorphian class. They will provide a continued infusion of new scientific input into the drug discovery process for identification of safer, more efficacious drugs with improved side effect profiles than the conventional therapies. Acknowledgments This work was supported by grants from the Austrian Science Fund (P11382, P12668, P15481, and P21350), European community (EPILA, QLK6-1999-02334), National Institute on Drug Abuse (N01DA-1-8816), and Drug Evaluation Committee of the College on Problems of Drug Dependence of the USA (N01DA-1-7725).

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Top Curr Chem (2011) 299: 93–119 DOI: 10.1007/128_2010_89 # Springer-Verlag Berlin Heidelberg 2010 Published online: 26 August 2010

14-Amino-4,5-Epoxymorphinan Derivatives and Their Pharmacological Actions John W. Lewis and Stephen M. Husbands

Abstract 14-Hydroxy-7,8-dihydromorphinone (oxymorphone) and its derivatives (oxycodone, naloxone, naltrexone) have become among the most important clinical agents to have been produced from opium. 14-Aminocodeinone and its 7,8-dihydro and morphinone derivatives are of more recent origin thanks to the work of Professor Gordon Kirby and his collaborators. The 14-amino parent compounds have proved of limited interest but their 14-acylamino- and 14-alkylamino derivatives have been extensively studied. The 40 -substituted cinnamoylamino-17-cyclopropylmethyl-7,8dihydronormorphinones, C-CAM and M-CAM are the best available selective MOR irreversible antagonists and the related dihydrocodeinone MC-CAM, 40 -chlorocinnamoylamino-17-cyclopropylmethyl-7,8-dihydronorcodeinone, is a long-acting MOR partial agonist with extended MOR-pseudoirreversible antagonist activity that could be a candidate for pharmacotherapy of opiate abuse/dependence. Keywords Agonist, Antagonist, Epoxymorphinan, MOR, Opioid

Contents 1 2

3

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94 Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 2.1 Synthesis of 14-Aminocodeinone, 14-Aminomorphinone and Their 7,8-Dihydro Analogues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 2.2 Synthesis of 14-Acylamino- and 14-Alkylamino-Codeinones and Morphinones . . . . 98 Pharmacology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100 3.1 Pharmacological Activities of 14-Acylaminomorphinones, 14-Alkylaminomorphinones, the Equivalent Codeinones and Their 17Cyclopropylmethyl Analogues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100 3.2 Pharmacological Actions of 14-Cinnamoylamino-17-Cyclopropylmethyl7,8-Dihydronormorphinones and Equivalent Codeinones . . . . . . . . . . . . . . . . . . . . . . . . . . 103

S.M. Husbands (*) and J.W. Lewis Department of Pharmacy and Pharmacology, University of Bath, Bath BA2 7AY, UK e-mail: [email protected]

94

J.W. Lewis and S.M. Husbands 3.3

Pharmacological Actions of 14-Pyridylacryloylaminodihydromorphinones and Codeinones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112 4 Pharmacological Actions of 14-Aminomorphindole and Derivatives . . . . . . . . . . . . . . . . . . . . . 114 5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116

Abbreviations CBM CPM DOR GTPgS i.c.v. i.p. KOR MOR s.c. TF TW

Cyclobutylmethyl Cyclopropylmethyl Delta opioid receptor Guanosine 50 -O-[gamma-thio]triphosphate Intracerebroventricular Intraperitoneal Kappa opioid receptor Mu opioid receptor Subcutaneous Tail flick Tail withdrawal

1 Introduction Naturally occurring and semisynthetic epoxymorphinans (1, 2) (Fig. 1) have proved to be of the utmost importance as analgesics for the treatment of moderate to severe pain as a result of their agonist actions at the mu opioid receptor (MOR). NMe

NMe R'

O RO

O OH

RO

NR' OH

O

O

RO

1a: R = Me, codeine 2a: R = Me, R'= H, hydrocodone 1b: R = H, morphine 2b: R=R'= H, hydromorphone 2c: R= Me, R'= OH, oxycodone 2d: R= H, R'= OH, oxymorphone Me + NMe NCPM OH OCOR

O HO

O O

4: Methylnaltrexone

MeO

5

O

Fig. 1 Epoxymorphinans and epoxymorphinones

O

3a: R= H, R'= CH2CH=CH2, Naloxone 3b: R= H, R'= CPM, Naltrexone 3c: R= Me, R'= CH2CH=CH2 3d: R= Me, R'= CPM NR' OCOR

MeO

5-9 a: R= (CH2)n CH3 b: R= CH=CHC6H5

O

O 6: R'= CH2CH=CH2 7: R'= CH2CH=C(Me)2 8: R'= CPM 9: R'= CBM

14-Amino-4,5-Epoxymorphinan Derivatives and Their Pharmacological Actions

95

The 17-N-allyl-(naloxone, 3a) and 17-N-cyclopropylmethyl (naltrexone, 3b) analogues of oxymorphone (2d) are the prototype opioid receptor antagonists with some selectivity for MOR. They have entered clinical practice as treatments for narcotic overdose (naloxone) and alcoholism or opioid abuse/dependence (naltrexone). The 17-quaternary derivative of naltrexone, methylnaltrexone (4) has recently been introduced into clinical practice as a treatment for opiate-induced bowel dysfunction [1]. Acylation of the 14-hydroxyl group of 14-hydroxycodeinone gave esters (5) (Fig. 1) in which antinociceptive potency increased with the length of the alkyl chain in the acyl group (5a) peaking in the heptanoyloxy derivative (5a, n = 5) which had 60 times the potency of morphine [2], a level of potency that was matched by the cinnamoyloxy derivative (5b) [3]; further increases in the chain length reduced potency [2]. A series of 14-O-acyl derivatives of 17-allyl-, 17-dimethylallyl-, 17-cyclopropylmethyl- and 17-cyclobutylmethyl-7,8-dihydronorcodeinones (6–9) (Fig. 1) were patented as safe analgesics with favourable dependence profiles [4]. The pentanoyloxy and hexanoyloxy derivatives in the 17-cyclopropylmethyl series (8a; n = 3,4) had the highest potency in the antiwrithing test in mice. Acylation of the 14hydroxyl group increased the antinociceptive potency of 3d by two orders of magnitude so that 8a (n = 3,4) were as potent as morphine. It was at this time that we, at Reckitt and Colman became interested in the work of Professor Gordon Kirby on Diels–Alder reactions of thebaine with nitroso derivatives with the objective of providing a viable synthesis of 14-aminocodeinone. This would allow access to a wide range of 14-acylamino- and 14-alkylamino-codeinones and morphinones which could be improvements on the available derivatives of 14-hydroxycodeinone and oxycodone. The first indication that derivatives of 14-aminocodeinone and 14-aminomorphinone could be accessible came from the reaction of thebaine with nitrosobenzene to yield a cyclic adduct (10a) which yielded 14b-phenylaminodihydrocodeinone (11a) on sequential hydrolysis and reduction (Scheme 1) [5]. Schwab then utilised this chemistry for the synthesis of a small series of 14-phenylaminodihydrocodeinones and derivatives (11) [6]. Some codeinone derivatives produced by these reactions were evaluated in antinociceptive assays, showing that the N-methyl derivatives (11a,b) had substantial activity in the writhing assay but only weak NR

10 1

NR NHAryl

NR Aryl

14

(i)

N

(ii), (iii) O

O MeO

5

9: R = Me or CPM

OMe

O MeO OMe 10a: R = Me, Aryl = Ph 10b: R = Me, Aryl = 4'-F-Ph 10c: R = CPM, Aryl = Ph 10d: R = CPM, Aryl = 4'-F-Ph

O MeO

O 11a: R = Me, Aryl = Ph 11b: R = Me, Aryl = 4'-F-Ph 11c: R = CPM, Aryl = Ph 11d: R = CPM, Aryl = 4'-F-Ph

Scheme 1 (i) Nitrosoarene, CHCl3, 57–96%; (ii) 1 N HCl, 86–91%; (iii) H2, 10% Pd/C, 22–57%

96

J.W. Lewis and S.M. Husbands

activity in the more demanding tail withdrawal (TW) assay (54 C water). In this assay the 17-cyclopropylmethyl analogues (11c,d) displayed modest morphine antagonist activity [6].

2 Synthesis 2.1

Synthesis of 14-Aminocodeinone, 14-Aminomorphinone and Their 7,8-Dihydro Analogues

Utilising the ability of thebaine (9) to react with electrophiles, the first viable synthesis of 14-aminocodeinone (13) was by initial nitration of thebaine with the mild nitrating agent tetranitromethane (Scheme 2). The reaction produced highest yields, though these are not recorded, in the presence of ammonia, but resulted in the generation of the ammonium salt of trinitromethane which is hazardous due to its thermal instability [7]. Nevertheless, 14b-aminocodeinone (13) and its 17-cyclopropylmethyl analogue (26) were prepared by this method in sufficient quantity to allow synthesis, characterization and pharmacological evaluation of close

NMe

NMe NO 2

NMe NH2 (ii), (iii)

(i) O MeO

O MeO

OMe

9

OMe OMe

12

(vi)

O MeO

O

13

NMe COPh N (ii)

O

(iv) O MeO

NMe

(v)

+

O O O MeO

14

MeO OMe

NMe NHOH

NMe NHOH

Cl

N

(v)

OMe

16

15a

OMe OMe

O MeO

15b

O

Scheme 2 (i) Tetranitromethane, methanolic ammonia, rt; (ii) Zn, NH4Cl, MeOH, reflux; (iii) 2 N HCl; (iv) 1-chloro-1-nitrosocyclohexane; (v) aqueous methanolic HCl, yield over steps iv, v, ii 35%; (vi) benzohydroxamic acid, tetraethylammonium periodate, 97%


97

to 300 opioid ligands listed in two patents [8, 9] resulting from the collaboration of Reckitt and Colman with Professor Kirby. At this time, efforts were also being made to extend the scope of the Diels–Alder chemistry of thebaine. Horsewood and Kirby [10] found that reaction of thebaine with 1-chloro-1-nitrosocyclohexane which, following hydrolysis with methanolic HCl, gave 14-hydroxyaminocodeinone 15b (Scheme 2), via adduct 14. This method was never widely adopted, only finding limited use within the medicinal chemistry community [11]. Similarly, 15b was also obtained through reaction of thebaine with benzohydroxamic acid in the presence of tetraethylammonium periodate, to generate the corresponding nitrosocarbonyl compound in situ, and thence the adduct 16, followed by hydrolysis in methanolic HCl [12]. Further improvements in the synthesis of 13, using in situ generated nitrosoformates, were made by the Kirby group (Scheme 3) [13]. This method has proven both more convenient and higher yielding than the earlier methods. In brief, 2,2,2trichloroethyl N-hydroxycarbamate was oxidised by sodium periodate in the presence of thebaine to yield adduct (17) which was converted into (18) before zinc powder mediated reduction to an amino acetal and hydrolysis to 14-aminocodeinone (13). Variations on this synthetic route remain the method of choice for accessing derivatives of 14-aminocodeinone. Improvements were made through process research at Reckitt & Colman (now Reckitt Benckiser). In this modification, adduct (17) was treated with ethylene glycol and trimethylorthoformate followed by acetyl chloride to yield ethylene ketal (19) (Scheme 3). This was reduced using zinc in ammonium carbonate with subsequent hydrolysis to the codeinone (13). This remains the current method of choice. The equivalent 14-aminodihydrocodeinone (20a) can be reached by direct hydrogenation of (13) (Scheme 4) [11, 14], giving an overall yield for the thebaine to 14b-aminodihydrocodeinone conversion in the range 50–60%. A more direct

NMe O

NMe

N O O

(i) MeO

O 9

OMe MeO

O 17

NMe N

CCl3 (ii)

OMe

O 18 O

MeO

O O OH

CCl3(iii), (iv) 13

O (vi), (iv)

(v) NMe N

MeO

O 19 O

O O OAc CCl 3 O

Scheme 3 (i) 2,2,2-Trichloroethyl N-hydroxycarbamate, sodium periodate, 0.5 M NaOAc, EtOAc, pH 6, 0 C, 82%; (ii) ethylene glycol, HCl, CH2Cl2, 95%; (iii) Zn, glycolic NH4Cl, 80%; (iv) HCl(aq), MeOH, 80%; (v) ethylene glycol, trimethylorthoformate, CH2Cl2 then AcCl; (vi) Zn, NH4CO3

98

J.W. Lewis and S.M. Husbands NMe NH2

O 13 NMe NH2

O O

O MeO R O 20a: R = H 20b: R = Me (ii)

O

CCl3

O

N

(ii)

(i)

MeO

NMe O

NMe NH2

MeO 21

Me OMe

N

NMe O O

N

NH2

Ph

O MeO

OH 23

O

O

O MeO

22

Me OMe

MeO

24: 7,8-etheno 25: 7,8-ethano

O

Scheme 4 (i) H2, 10% Pd/C, MeOH; (ii) H2, 10% Pd/C, MeOH, NaOAc/HOAc buffer, 73% (21 ! 20b), 82% (22 ! 20b)

method, involving catalytic reduction of the adduct (21), is possible with 20b, having a 5b-methyl group [15]. This latter method has also been updated by Archer and colleagues [16] in which benzyl N-hydroxycarbamate replaces the trichloroethyl ester to furnish adduct (22), with catalytic reduction giving dihydrocodeinone (20b) in excellent yield (76% from thebaine) (Scheme 4). This direct route to 20b does not work reliably for 20a, with treatment of 17 under the same conditions leading to hydrogenolysis of the oxide bridge and formation of the C4-phenol (23) [17]. Access to the equivalent 17-cyclopropylmethyl compounds (24, 25) uses the same chemistry, but starting from 17-cyclopropylmethylnorthebaine. The yield of around 20% for the synthesis of 25 from 17-cyclopropylmethylnorthebaine using this method is substantially reduced compared to the 17-methyl series where the yield is typically 50–60%. The corresponding morphinones are then reached by boron tribromide mediated 3-O-demethylation [18, 19].

2.2

Synthesis of 14-Acylamino- and 14-Alkylamino-Codeinones and Morphinones

Alkyl and acyl derivatives (28, 30) of 14-aminocodeinones (13, 26) and dihydrocodeinones (20a, 27) are readily prepared; acylation is achieved under standard conditions of acid chloride or acid anhydride in the presence of an organic base, while alkylation can be achieved directly with an alkyl halide or by acylation and then lithium aluminium hydride reduction (Scheme 5). In this latter method the 6-keto group is protected as an acetal [8, 9, 18, 19]. For direct alkylation with unactivated alkyl halides, carrying out the reaction in a sealed tube at 120 C has

14-Amino-4,5-Epoxymorphinan Derivatives and Their Pharmacological Actions NR NHCOR'

99 NR NHCOR'

(ii)

NR NH2

(i)

O

29

HO

NR NHCH2R'

(iii) O O MeO 13: R=Me, 7,8-etheno 20a: R=Me, 7,8-ethano 26: R=CPM, 7,8-etheno 27: R=CPM, 7,8-ethano

O O

28

MeO

O

NR NHCH2R' (ii)

O 30

MeO

(iv)

O O

HO

31

O

(vi)

NR NH2

NR NHCH2R' (i), (v) O

O MeO

O

32

O

O MeO

33

O

Scheme 5 (i) R0 COCl or R0 COOCOR0 , NEt3, 40–90%; (ii) BBr3, CH2Cl2, 50–80%; (iii) R0 CH2X (X = halide), NaHCO3, 35–80%; (iv) ethylene glycol, HCl, CH2Cl2; (v) LiAlH4, THF, 60%

N

EtO

N H N

NH2

P

(i) O MeO

O 27

O

MeO

O 34

O

O

N H N

OEt (ii)

O MeO

O 35

Cl

O

Scheme 6 (i) (EtO)2P(O)CH2COCl, THF, 60%; (ii) LDA, THF then 4-chlorophenylacetaldehyde, 28%

proven beneficial [20]. The corresponding morphinones (29, 31) are then reached by boron tribromide mediated 3-O-demethylation [18, 19]. Morphinones can be converted into alternative 3-O-alkyl ethers (41), in high yield, through the use of an appropriate alkyl halide, potassium carbonate in wet acetone [21]. The instability of 4-(40 -chlorophenyl)-2-butenoic acid led to an alternative strategy for the preparation of (35) (Scheme 6) [18]. The protocol involved Horner–Wadsworth–Emmons-type chemistry, initially acylating with diethoxyphosphorylacetyl chloride before treatment with lithium diisopropylamide and 4-chlorophenylacetaldehyde.

100


3 Pharmacology 3.1

Pharmacological Activities of 14-Acylaminomorphinones, 14-Alkylaminomorphinones, the Equivalent Codeinones and Their 17-Cyclopropylmethyl Analogues

With the availability of a viable synthesis of 14-aminocodeinone (13) and the closely related analogues 20a, 26 and 27, Reckitt and Colman in collaboration with Professor Kirby set about the synthesis of a whole range of 14-alkylamino-, 14-arylalkylamino- and 14-acylaminocodeinones and equivalent morphinones as well as the 7,8-dihydro analogues and the N-cyclopropylmethyl equivalents. The synthesised compounds were listed in two patents, US 4,241,066 and US 4,241,067 [8, 9]. Many of them were evaluated in a primary screen for MOR activity, in vitro in the mouse vas deferens assay and in vivo in rat antinociceptive assays (Tables 1 and 2). In the in vivo assays tail pressure was the nociceptive stimulus for agonist activity, whereas for morphine antagonism warm water (55 C) was used with tail withdrawal as the end point. MOR agonist potency increased in the series of acylaminomorphinones (36c) (Fig. 2) peaking at the hexanoylamino derivative [R00 = (CH2)4CH3] which was over 4000-times more potent than normorphine in vitro, though only 100-fold more potent than morphine in vivo (Table 1). This structure–activity relationship closely matches that found in the 14-O-acylcodeinones [2]. In the alkylaminomorphinone series (37c) the peak of activity was reached with the hexylamino derivative [R00 = (CH2)4CH3] which was 1,000 times more potent than normorphine in vitro and with the same potency advantage over morphine in vivo. When a phenyl group was located at the end of an acylamino side chain, peak activity was reached in the phenylpropiolylamino derivative [36c, R00 = Ph(CH2)2] which was over 700 times more potent in vitro and 200 times more potent in vivo than normorphine and morphine, respectively (Table 1). In the 14-acylamino-17-cyclopropylmethylnormorphinone series (36d) the lower members (formylamino-, acetylamino- and propanoylamino-) were potent MOR antagonists in vitro equal or more potent than naloxone but with much lower activity in vivo (Table 2). This may be due to relatively rapid metabolic hydrolysis to the hydrophilic 14-amino parent (38) which had a similar loss of in vivo activity when compared to its in vitro potency. From the 14-butanoylamino derivative through to heptanoylamino- [36d: R00 = (CH2)2CH3 through (CH2)5CH3] the N-cyclopropylmethyl derivatives were MOR partial agonists in vitro with increasing potency as agonists but retaining potent antagonist potency. MOR agonism in vivo peaked at hexanoylamino- but surprisingly none of the ligands had significant MOR antagonism in vivo (Table 2). The 14-alkylamino-17-cyclopropylaminonormorphinone series (37d) in vitro showed substantial differences from the acylamino equivalents (36d). From methylamino- through to hexylamino- they had no agonist activity in vitro but were potent


101

Table 1 In vitro and in vivo agonist activity of compounds of structures 36 and 37 (Fig. 2) In vitro potency In vivo potency R00 times normorphinea times morphineb Me 36a 0.17 0.1 n-Pr 2.8 2.1 n-Bu 3.8 3.7 n-Pent 27 77 n-Hex 31 168 n-Hept 8.9 36 36c

37a

37c

Me n-Pr n-Bu n-Pent n-Hex n-Hept Ph CH2Ph (CH2)2Ph (CH2)3Ph (CH2)4Ph

0.4 32 191 4000 628 394 38 272 744 490 32

0.44 12 56 117 82 61 18 148 222 7.0 11

Me Et n-Pr n-Bu n-Pent n-Hex n-Hept

0.2 0.15 0.25 0.9 8.1 30 46

0.67 0.73 2.4 2.0 6.6 73 9.4

n-Pr 20 n-Bu 39 n-Pent 220 n-Hex 1,000 n-Hept 538 n-Oct 289 CH2Ph 32 (CH2)2Ph 3370 (CH2)3Ph 1,500 508 (CH2)4Ph (CH2)5Ph 329 a Determined in the mouse vas deferens b Determined in rat tail pressure test

65 11 297 980 408 153 34 2512 ND ND ND

antagonists. The heptylamino- derivative retained potent antagonism in vitro but also had mouse vas deferens agonist activity of greater potency than normorphine. It was also active in the rat tail pressure test of analgesic activity (Table 2). From the initial evaluation of the large number of 14-acylamino- and 14-alkylaminocodeinones and morphinones synthesised in the Reckitt and Colman programme, it became clear that the C14 side chains of particular interest were those with an aryl group joined to C14 by an alkyl group of two to four carbon atoms with three carbon chain compounds having special qualities. Lewis et al. [14] revealed

102


Table 2 In vitro and in vivo activity of compounds of structure 36 and 37 (Fig. 2) Agonist in vitro Antagonist Agonist in vivo Antagonist R00 potency times in vitro potency potency times in vivo potency normorphinea times naloxoneb morphinec times naloxoned H 36b 0.1 ND 3.2 mg/kg) substantially depressed the maximum effect of morphine with the highest dose (32 mg/kg) also producing the suggestion of suppression of the effect of fentanyl. This is consistent with the designation of fentanyl as a more efficacious MOR agonist than morphine and suggested an irreversible MOR antagonist effect of C-CAM. The highest dose of C-CAM (32 mg/kg) antagonised the antinociceptive effect of morphine for up to 8 days. The irreversible nature of C-CAM’s MOR antagonism was confirmed in vivo and in binding experiments by Burke et al. [26]. However, when mouse brain membranes were incubated with [3H]C-CAM followed by precipitation of the protein, no specific radiolabelling of receptor protein was seen, suggesting C-CAM did not bind covalently to MOR. Following the initial evidence of C-CAM’s ability to rank efficacy of MOR agonists [25], it was reported that C-CAM could be utilised to give efficacy and apparent affinity estimates for MOR agonists in mice [27], rhesus monkeys [28] and squirrel monkeys [29]. Relative efficacies of MOR agonists in drug discrimination assays using C-CAM have also been reported in pigeons [30] and rats [31].


105

Though the chloro-substituted 14-cinnamoylamino derivatives were the primary focus of attention during the 1990s, the analogue of C-CAM, 40 -methylcinnamoylaminodihydromorphinone (M-CAM, 40f) when compared to C-CAM and b-FNA was an even more impressive MOR irreversible antagonist. This was shown in a direct comparison of the three ligands [32]. Neither C-CAM or M-CAM had significant antinociceptive activity in the acetic acid-induced writhing assay and showed very low levels of stimulation of [35S]GTPgS binding in MOR and delta opioid receptors (DORs) expressed in C6 cells and in kappa opioid receptors (KORs) expressed in CHO cells. b-FNA (32 mg/kg) effectively suppressed writhing (90% response) and in CHO-hKOR cells b-FNA stimulated [35S]GTPgS binding as a KOR partial agonist [33, 34]. M-CAM was more effective than C-CAM and b-FNA in suppressing MOR antinociceptive activity in TW (55 C water) and in the writhing assay the order of potency in shifting the morphine dose–response curve rightwards was M-CAM > C-CAM > b-FNA. In this assay, which has a large receptor reserve, no flattening of the agonist dose–response curves by the irreversible antagonists was observed; M-CAM also demonstrated better selectivity for MOR over DOR and KOR, than C-CAM and b-FNA [32]. Woods et al. wrote a monograph on the pharmacology of methoclocinnamox (MC-CAM, 40a) with comparison made to buprenorphine to determine whether it too could be of interest as a pharmacotherapy for opioid abuse [35]. MC-CAM and its parent phenol C-CAM had very similar high affinities (IC50 1.0 and 0.6 nM) for tritiated etorphine labelled binding sites. In the mouse vas deferens assay MC-CAM inhibited the electrically stimulated twitch with nanomolar potency, an effect that could be prevented by pre-incubation with naltrexone. In the [35S]GTPgS assay MC-CAM was not an agonist at any of the three opioid receptors, but was an antagonist with approximately tenfold selectivity for MOR over KOR and DOR [19]. In mouse antinociceptive assays MC-CAM was inactive in the TF and the TW assays. It was active in the acetic acid-elicited writhing assay in which it was fully active at a dose of 3.2 mg/kg, the effect of which lasted for at least 8 h. MC-CAM’s antiwrithing effect was prevented by naltrexone and b-FNA but not by the KOR antagonist norBNI or by the DOR antagonist naltrindole. However, when naltrexone was used to reverse the antinociceptive effect of MC-CAM it was found to be effective for only the first hour after naltrexone administration. Thereafter the reversal effect of naltrexone was insignificant, representing conversion of a reversible agonist action of MC-CAM into antagonist-resistant agonism [35]. The antagonist actions of MC-CAM in the antinociceptive assays were investigated when it was administered 24 h before opioid receptor agonist challenge, i.e. when all agonist effects had dissipated. In TW (50 C water), a high dose (32 mg/kg) of MC-CAM substantially flattened the morphine dose–response curve and effectively antagonised morphine even 4 days after its administration. In the acetic acid-elicited writhing assay this dose shifted the morphine dose–response curve threefold to the right but had no effect on the curves of the selective KOR agonist U69593 and the selective DOR agonist BW373U86 [35]. The agonist and antagonist effects of MC-CAM were investigated in a TW procedure in rhesus monkeys. Using 55 C water MC-CAM was inactive but at

106


50 C it had slow onset and reached the maximum possible effect after 2 h; this was maintained for 90 min when it declined to reach control levels at the 5th hour. MC-CAM’s antagonist effect was studied in a dose of 1 mg/kg against the shortacting MOR agonist alfentanil in 55 C water over 11 days. The antagonist effect of MC-CAM was demonstrable at 24 h pretreatment when the shift of the alfentanil dose–response curve was tenfold, peaked at 48 h when there was evidence of the flattening of the agonist’s dose–response curve and was still evident after a week. After 11 days alfentanil’s analgesic actions were fully restored [35]. MC-CAM like other MOR agonists was an effective reinforcer using a self-administration procedure that required relatively low capacity of the reinforcer [35, 36]. A dose of 1 mg/kg of MC-CAM was also able to reduce the reinforcing potency of the high efficacy MOR agonist alfentanil initially by 30-fold but significantly for 72 h. A higher dose (3.2 mg/kg) was also able to reduce cocaine self-administration but this effect was of short duration [35]. In morphine-dependent rhesus monkeys, MC-CAM substituted for morphine in withdrawn animals at a dose of 0.8 mg/kg but in non-withdrawn animals the same dose of MC-CAM produced signs of withdrawal slowly over a 3-day period; these signs were incompletely suppressed by regular morphine injections [35]. Although in the tests that had been applied to MC-CAM, buprenorphine showed close similarity, there were some notable differences. In particular MC-CAM’s opioid antagonist effects were MOR-selective whereas buprenorphine also had KOR and DOR antagonism. In rhesus monkeys MC-CAM also appeared to have greater MOR agonist efficacy than buprenorphine in antinociceptive assays and, at least initially, unlike buprenorphine it suppressed morphine abstinence. Based on these similarities to, and differences from, buprenorphine, the ligand deserves consideration as a treatment for opioid abuse. The Lewis and Husbands group have studied in some detail structure–activity relationships relating to the 14-cinnamoylaminodihydrocodeinones and morphinones [37]. In light of the possibility that the delayed morphine antagonism of MC-CAM could be due to its metabolism to C-CAM, a range of alternative phenolic ethers (41) (Fig. 4) of C-CAM were prepared and evaluated [21]. The study showed that, when compared to the methyl ether (41a, MC-CAM), the cyclopropylmethyl Cl N

H N

O O R'O

Fig. 4 Ethers of C-CAM

O

41 (a) R' = Me (b) R' = CPM (c) R' = CH2CH=CH2 (d) R' = CH2CN (e) R' = (CH2)2CH3 ( f ) R' = CH2CCH


107

ether (41b) was devoid of MOR agonist effects in both mouse tail withdrawal and antiwrithing assays. Conversely, four of the other ethers (41c–f) had more agonist effect than MC-CAM since they were active in the TW assay (50 C water) with the propargyl ether (41f) showing greatest activity, with potency at least equal to morphine, which was confirmed in the TF assay. All the new ethers had morphine antagonist activity in TW when they were administered 24 h before morphine and when their agonist effects had waned. The cyclopropylmethyl ether (41b) at 32 mg/kg flattened the morphine dose–response curve for more than 48 h and had antagonist effects for greater than 6 days. The overall conclusion from the study was that the results did not support the thesis that the delayed MOR antagonist activity of MC-CAM is due to its metabolism to C-CAM since the metabolic O-demethylation of MC-CAM should be a substantially faster process than other dealkylations and make MC-CAM a better MOR antagonist than the other ethers. A significant finding was the effect of removal of the 3-phenolic hydroxyl group from C-CAM to give DOC-CAM (42, Fig. 5) [38]. In several series of opioids it has been shown that the 3-phenol has higher MOR affinity than the corresponding deoxy (3-H) and 3-methoxy analogues. However, in the 14-cinnamoylamino series DOC-CAM, MC-CAM and C-CAM have similar affinity for MOR in binding assays (Kis 0.54 nM, 0.46 nM and 0.25 nM, respectively) and DOC-CAM and C-CAM have similar potencies in antagonising the MOR agonist fentanyl in the [35S]GTPgS assay (Kes 0.28 nM and 0.37 nM, respectively). The major difference between DOC-CAM and C-CAM lies in the lack of irreversible character in DOCCAM’s in vivo MOR antagonist activity. Thus it appears that binding of both the 3-oxygen atom and the cinnamoyl group is required for irreversible MOR antagonist activity. Investigation of the influence of the chain length, position of the alkene group and reduction of the alkene and amide groups in the side chain of analogues of C-CAM was reported by Rennison et al. in compounds of general structure 43 (Fig. 6) [18]. MOR binding affinity for compounds having three carbon chains in the C14 substituent was substantially higher than for equivalent four or two carbon chains. The codeinones (43: R0 = Me) showed some selectivity for MOR over KOR

Cl N H N

O O O

Fig. 5 3-Deoxyclocinnamox

42 : DOC-CAM

108


Cl

43 (a) X = COCH=CH

N

(b) X = COCH2CH2

H N

X

(c) X = CH2CH=CH (d) X = (CH2)3 (e) X = COCH=CHCH2 ( f ) X = COCH2CH=CH (g) X = CO(CH2)3

O R'O

O

(h) X = CH2CH=CHCH2 ( i ) X = (CH2)4 ( j ) X = COCH2 (k) X = (CH2)2

Fig. 6 14-Aminomorphinones with varying C14-side chains

and DOR whereas the morphinones (43: R0 = H) had high affinity for all opioid receptors. 43b and 43d with saturated C14 side chains had exceptional potency as non-selective opioid antagonists in vivo [39]. At a high dose (32 mg/kg s.c.) with 24 h pretreatment, 43b suppressed morphine’s antinociceptive effect in TW to the extent that 320 mg/kg morphine, the highest dose tested, had only 35% of the maximum possible effect whereas the ED100 for morphine in this assay was 100 mg/kg. The effect of 43d was even more dramatic; 32 mg/kg of 43d administered 24 h before morphine totally suppressed the morphine dose–response curve. The pseudoirreversible MOR antagonist effect of 43b is equivalent to that of C-CAM (43a: R0 = H) whereas that of 43d is more impressive than C-CAM’s and compares with that of M-CAM [32]. Whereas the codeinones and morphinones with three carbon C14 chains (43a–d) were all potent antagonists for all three opioid receptors in [35S]GTPgS assays, the four carbon chain analogues (43e–i) were MOR antagonists, DOR antagonists or partial agonists and KOR antagonists or partial agonists but with much lower potency. The two carbon chain codeinones and morphinones (43j–k) were the only ones to have MOR partial agonist activity; they were also low potency DOR and KOR partial agonists. In a study using the [35S]GTPgS assay and looking at the interaction of DAMGO with ligands having side chains with two and four carbon atoms as well as C-CAM (43a, R0 ¼ H), the evidence for irreversible MOR binding showed that all three chain lengths were associated with such binding. However, the phenylacetylmorphinone (43j, R0 ¼ H) in the TW assay had substantial MOR agonist activity, greater than was expected from its low efficacy MOR partial agonism in vitro. There was no evidence of delayed MOR antagonism in the TW assay. Thus there appears to be inconsistency in the in vivo data when compared to the in vitro profile [18]. Nieland et al. [19] studied the effect of orientation in the aromatic ring of the cinnamoylamino side chain (44, Fig. 7). It was realised early that a 40 -chloro or 40 -methyl substituent as in C-CAM, MC-CAM or M-CAM was associated with powerful irreversible MOR antagonism; the same two substituents when placed in


NR'

4'

H N

3' 2'

X

O O O

RO

109

44 a: X = H b: X = 4'-Cl c: X = 4'-Me d: X = 4'-NO2 e: X = 2'-Cl f : X = 2'-Me g: X = 2'-NO2

Fig. 7 Substituents in the cinnamoylamino side chain R2 N O O O R1O

O

45 (a) R1 = Me, R2 = H (b) R1 = Me, R2 = Cl (c) R1 = Me, R2= Me (d) R1 = R2 = H (e) R1 = H, R2 = Cl HO ( f ) R1 = R2 = Me

N

N

O(CH2)3Ph

OH

O O 3b: naltrexone

O HO

46: PPN

O

Fig. 8 14-O-Substituted derivatives of naltrexone

the 20 -position produced ligands with very much higher MOR efficacy. In [35S] GTPgS assays with the 20 -substituted 17-cyclopropylmethyl ligands (44, R0 ¼ CPM) this higher efficacy was more apparent for KOR and DOR activity than for MOR activity, whereas for the 17-methyl ligands (44, R0 ¼ Me) it was true for all opioid receptors. A limited amount of in vivo data for the 17-cyclopropylmethyl 20 - and 40 -nitro derivatives (44d, g, R0 ¼ CPM) suggested that the effect of orientation of the nitro group was different to that of the chloro and methyl substituents. Thus the 40 nitrodihydrocodeinone (44d, R ¼ Me) had antinociceptive activity in the TW substantially greater than that of the 20 -nitro isomer (44 g, R ¼ Me). The 40 nitrodihydromorphinone (44d, R ¼ H) had higher efficacy in TW than any other 17-cyclopropylmethyldihydromorphinone tested, including the unsubstituted derivative (44a, R ¼ H) and the 20 -chloro and 20 -methyl (44e, f, R ¼ H) analogues. 44d (R ¼ H, R0 ¼ CPM) was also an impressive delayed morphine antagonist of long duration. The differences between chloro/methyl and nitro substitution seem to reflect the greater lipophilicity of the halo and methyl groups though the reason for this particular differentiation is unclear [19]. 40 -Chloro-, 40 -methyl- and unsubstituted 14-O-cinnamoyl derivatives of naltrexone (45, Fig. 8) have also been evaluated for comparison with the equivalent amides, C-CAM, M-CAM and MC-CAM [40]. In in vitro assays only limited efficacy, predominantly at KOR, was observed for the codeinones (45a–c), while the morphinones (45d–f) were all antagonists of greater potency than their parent, naltrexone (3b). In vivo the morphinones (45d–f) were morphine antagonists with irreversible characteristics, but of shorter duration than their amide counterparts. As in the amides, an unsubstituted cinnamoyloxy ring was associated with higher efficacy than the 40 -substituted analogues. It is interesting to compare the activities

110


of the unsubstituted morphinone (45d) with the phenylpropyloxy derivative (46, PPN) as both have a 3-carbon chain linking the aryl ring with the C14 oxygen. The ether (46, PPN) was a potent agonist, 400–600 times more potent than morphine in the hotplate and TF assays [41], whilst the ester (46d) displayed much lower agonist potency and only in the writhing assay where the nociceptive stimulus is of substantially lower intensity than in the thermal assays [40]. The group of Archer and Bidlack produced a substantial body of work on 14b-40 nitrocinnamoylaminocodeinone (47a) and morphinone (47b) and the equivalent dihydrocodeinone (48i) and dihydromorphinone (48j) including analogues with 5bmethyl substitution (48a–f) (Fig. 9). Two compounds received special attention. MET-CAMO (48a) and N-CPM-MET-CAMO (48b) having 5b-methyl substitution had high affinity and MOR selectivity in binding assays and when administered i.c.v. antagonised morphine selectively in TW for up to 72 h [42]. Comparisons between equivalent 40 -nitro- and 40 -chlorocinnamoylamino ligands were also made. MET-Cl-CAMO (48c) and N-CPM-MET-Cl-CAMO (48d) were long-term MOR antagonists devoid of agonist properties when administered i.c.v. in TW with 55 C water [43]. There were only minor differences when compared with MET-CAMO (48a) and N-CPM-MET-CAMO (48b) [42]. The 40 -chlorocinnamoylaminodihydrocodeinones CAM (48e) and MC-CAM (48g) were compared with the 40 -nitro analogues CACO (48f) and N-CPM-CACO (48h). Again the chloro and nitro derivatives had similar long-term MOR antagonist profiles but whereas the nitro derivatives had short-term agonism when administered i.c.v. in the 55 C TW assay, CAM and MC-CAM were devoid of agonist activity in this assay [24].

NO2 NMe

R3

H N

NR

O

O

O RO 47a: R=Me 47b: R=H

H N

O

O

R1O

R2

O

48 (a) R = R2 = Me, R1 = H, R3 = NO2 MET-CAMO (b) R = CPM, R1 = H, R2 = Me, R3 = NO2 N-CPM-MET-CAMO (c) R = R2 = Me, R1 = H, R3 = Cl MET-Cl-CAMO (d) R = CPM, R1= H, R2 = Me, R3 = Cl N-CPM-MET-Cl-CAMO (e) R = R1 = R2 = Me, R3 = Cl CAM (f) R = R1 = R2 = Me, R3 = NO2 CACO (g) R = CPM, R1 = Me, R2 = H, R3 = Cl MC-CAM (h) R = CPM, R1 = R2 = Me, R3 = NO2 N-CPM-CACO (i) R=R1=Me, R2=H, R3=NO2 (j) R=Me, R1=R2=H, R3=NO2

Fig. 9 The CAMO and CACO derivatives


111

For both CAM and MC-CAM we found MOR agonist activity when they were administered parenterally, though in the case of MC-CAM it was observed only in the antiwrithing assay and not in TW [35]. CAM (48e) on the other hand, with parenteral administration in TF, was a powerful antinociceptive agent 50 times more potent than morphine [19]. The disparity between the MOR agonist effects of CAM (48e) in vitro and in vivo when administered peripherally and the agonist-free MOR antagonism when administered i.c.v. strongly suggest that the cinnamoylaminocodeinones do not owe their delayed MOR antagonist effects to metabolism to the morphinones [14] since direct administration into the brain should minimise the opportunity for extensive metabolism. Archer et al. also studied 14-thioglycolamidonordihydromorphinone derivatives (Fig. 10). TAMO and N-CPM-TAMO were originally given the monomer structure (49) [44–47] but it was subsequently realised that the thioglycolamides undergo rapid oxidation in air and on chromatography columns to the dimeric structures (50) [48]. Both ligands were shown to have wash-resistant binding to MOR, the nature of which showed that it was due to the formation of a covalent bond to MOR. In the TW assay TAMO (50a) produced antinociceptive effects lasting 5 h when given i.p. and 2.5 h when given i.c.v. [45, 49]. These effects were antagonised by b-FNA selectively showing that the antinociceptive actions of TAMO are MOR-mediated. From 8 h to 48 h after i.c.v. administration TAMO selectively antagonised the antinociceptive action of morphine in TW. In a study of the behavioural effects of TAMO, its effects in rhesus monkeys on schedule-controlled behaviour and thermal nociception were investigated [49]. TAMO produced effects typical of MOR agonists in both assays, i.e. decrease in food maintained responding and increase in tail withdrawal latency; both effects were inhibited by the MOR antagonist quadazocine. Pretreatment with TAMO at 24 h failed to inhibit the behavioural effects of the MOR agonist fentanyl but it did produce a small rightward shift in the morphine dose–response curve. This lack of substantial MOR antagonism in monkeys contrasts with the irreversible MOR antagonist effect in mice. NR H N

NR H N

R2

O

HO

N OH

HO

O

O

O 49

S

O

O HO

S

SH

H RN N

O 51: β-FNA NHCOCH=CHCO2CH3

Fig. 10 TAMO and derivatives

O 50a: TAMO: R=Me 50b: N-CPM-TAMO: R=CPM 50c: N-CBM-TAMO: R=CBM

O O

OH

112


The explanation could be simply species differences but is more likely related to the route of administration – i.c.v. in mice, parenteral in monkeys. N-CPM-TAMO (50b) was evaluated in TW assays by i.c.v. administration. It had no antinociceptive activity but was an irreversible MOR antagonist. In the antiwrithing assay N-CPM-TAMO had antinociceptive activity mediated by KOR [46]. It was evaluated together with b-FNA (51) and N-CPM-MET-CAMO (48b) to determine whether they had any effect on morphine-induced antinociceptive tolerance before they showed antagonism of the antinociceptive action of morphine in the tail withdrawal assay (55 C water); all opioids were administered i.c.v. [50]. In keeping with the rapidly produced wash-resistant binding to MOR, the antagonists inhibited morphine tolerance from 2.3 to 7 h post-administration whereas antagonism of morphine-induced antinociception was not observed until 8 h post-administration. It was concluded that long-term antagonism of MOR by irreversible antagonists is more complex than their direct binding to the MOR binding site. The cyclobutylmethyl derivative N-CBM-TAMO (50c) has also been reported [51, 52]. It showed wash-resistant binding to MOR and KOR but only to MOR did the binding characteristics indicate that the binding was covalent. In TW (55 C water) with i.c.v. administration N-CBM-TAMO had antinociceptive activity that was partially blocked by both b-FNA (MOR) and norBNI (KOR). It also had longterm antagonism of both MOR and KOR but only the antagonism of morphine (MOR) was irreversible [51]. A dose of N-CBM-TAMO (12 mg/kg i.p.) suppressed cocaine and morphine self-administration for 2–3 days with no suppression of water intake [52].

3.3

Pharmacological Actions of 14Pyridylacryloylaminodihydromorphinones and Codeinones

The effect of orientation of nitro groups in the cinnamoyl aromatic ring was that the para (40 -) substituent was associated with higher MOR efficacy and more pronounced long-term MOR antagonism than the ortho (20 -) substituent [19]. In order to throw light on the possible steric effects associated with this difference the isomeric pyridylacryloylamino-dihydrocodeinones (52a,c–54a,c) and morphinones (52b,d–54b,d) were synthesised since they would have electronic character equivalent to the nitrocinnamoylamino derivatives, but not the steric bulk (Fig. 11) NR H N

NR H N

N

O

O R1O

O 52a: R=R'=Me 52b: R=Me, R'=H 52c: R=CPM, R'=Me 52d: R=CPM, R'=H

O

O

O R1O


Fig. 11 Pyridylacryloylamino derivatives

N

NR H N

N

O R1O



113

[17, 53]. The new ligands were evaluated in receptor binding assays and in the stimulation of [35S]GTPgS binding functional assay, in both cases using recombinant MOR, DOR and KOR transfected into Chinese hamster ovary (CHO) cells (Tables 3 and 4) [33]. All the isomeric pyridylacryloylamino derivatives had high affinity for all three opioid receptors with little selectivity (data not shown). In the functional assays the dihydrocodeinones (52a–54a) showed a clear MOR > DOR > KOR pattern of agonist potency. The dihydromorphinones (52b–54b) were potent agonists for all three receptor types; the dihydrocodeinones (52a–54a) were substantially less potent (Table 3). All of the 17-cyclopropylmethyl compounds (52c,d–54c,d) were MOR antagonists in this assay. The dihydrocodeinones (52c–54c) were low potency KOR and DOR partial agonists whereas the dihydromorphinones (52d–54d) were KOR and DOR antagonists, with the exception of the 40 -isomer (54d) which had substantial KOR partial agonist activity (Table 4). Among the 14-pyridylacryloylaminocodeinones and morphinones there was not a consistent relationship between 20 -, 30 - and 40 -isomers in terms of MOR agonist efficacy; in the 17-methyldihydrocodeinones (52a–54a) the relationship was 20 > 30 > 40 , whereas with the corresponding dihydromorphinones (52b–54b) the reverse was true. It was therefore difficult to draw comparisons with the

Table 3 Agonist activity of 14-pyridylacryloylaminodihydromorphinones and codeinones in the [35S]GTPgS assay N-position MOR DOR KOR R R1 EC50/nM:% stim’ EC50/nM: EC50/nM: % stim’ % stim’ 20 Me Me 52a 132:61 12.9:90 4.9:102 20 H Me 52b 1.2:59 3.0:89 0.6:66 Me Me 30 53a 147:88 32.6:88 5.2:98 H Me 53b 30 5.9:67 3.1:104 0.5:66 Me Me 40 54a 68.1:100 29.0:87 19.6:77 H Me 54b 40 0.4:104 1.2:122 0.5:104 % stimulation relative to the standard agonists DAMGO (MOR), DPDPE (DOR) and U69593 (KOR) Table 4 Activity of 17-CPM-14-pyridylacryloylaminodihydronormorphinones and codeinones in the [35S]GTPgS assay N-position MOR DOR KOR R R1 EC50/nM:% stim’ EC50/nM:% stim’ EC50/nM:% stim’ or [Ke/nM] or [Ke/nM] or [Ke/nM] 13.9:57 193:39 [1.45] CPM Me 20 52c [0.057] [0.41] [0.171] 52d CPM H 20 40.1:48 102:33 [2.86] CPM Me 30 53c 30 [0.105] [0.23] [0.192] 53d CPM H 13.7:93 235:43 [2.98] CPM Me 40 54c 0.3:59 40 [1.42] [0.18] 54d CPM H % stimulation relative to, or antagonism of, the standard agonists DAMGO (MOR), DPDPE (DOR) and U69593 (KOR)

114


nitrocinnamoylamino derivatives, to give information on the contribution of steric and electronic effects to structure–activity relationships. Opioid ligands which exhibit KOR agonism or DOR antagonism have been shown to inhibit some of the behavioural actions of cocaine [54, 55]. A compound with both actions might be expected to have potential as a treatment for cocaine abuse, though a full KOR agonist is unlikely to be acceptable on account of its likely psychomimetic effects. The 17-cyclopropylmethyl-40 -pyridylacrylamino derivative (54d), with its in vitro profile of KOR partial agonism (EC50 0.3 nM; 59% of efficacy of U69,593) and MOR/DOR antagonism (Ke 0.18 nM vs DAMGO; Ke 1.42 nM vs DPDPE) may be effective against cocaine and be relatively free from unwanted side effects.

4 Pharmacological Actions of 14-Aminomorphindole and Derivatives 14-Amino-7,8-dihydromorphindole (55a, Fig. 12) and a selection of its 14-alkylamino (56) and 14-acylamino (57) derivatives were prepared from 20a by Fischer indole reaction, followed by acylation or alkylation of the amino group and 3-O-demethylation [20]. Aminomorphindole (55a) had high affinity binding to DOR and very much lower binding to KOR and particularly MOR, giving DOR selectivity comparable to oxymorphindole (58a) (Table 5). In the [35S]GTPgS functional assay it showed partial agonist activity at DOR and low potency KOR antagonism (Table 5). Alkylation (56a,b) of the 14-amino group increased KOR and particularly MOR affinity to reduce sharply DOR selectivity. In the phenethylamino- (56c) and phenylpropyl- (56d) amino derivatives, though DOR affinity remained high, KOR and MOR affinity were further increased to reduce DOR selectivity even further. In the [35S]GTPgS assays the alkylamino and phenylalkylamino derivatives all had DOR partial agonist activity of similar efficacy but higher potency than the parent aminomorphindole. The alkylamino derivatives (56a,b) had good DOR selectivity over KOR and MOR in potency terms. The phenylalkylamino morphindoles (56c,d) were still DOR selective but the phenethyl derivative (56c) also had high MOR potency and efficacy and could be of interest as an analgesic with

NR NH2

NMe H N

NMe H N

R

R

NR OH

O O HO 55a: R=Me 55b: R=CPM

N H

O HO

N H

56a: R=Me 56b: R=(CH2)3CH3 56c: R=CH2Ph 56d: R=(CH2)2Ph

Fig. 12 14-Aminomorphindole and derivatives

O HO

N H

57a: R=Me 57b: R=(CH2)3CH3 57c: R=CH2Ph 57d: R=(CH2)2Ph

O N HO H 58a: R=Me, OMI 58b: R=CPM, NTI


115

Table 5 Activity of 14-aminomorphindole and derivatives in binding and [35S]GTPgS assays DORb KORb MORb Bindinga DOR, Selectivity Selectivity EC50/nM:% EC50/nM:% EC50/nM:% DOR/ Ki/nM DOR/ stim’ or [Ke/nM] stim’ or [Ke/nM] stim’ or [Ke/nM] KOR MOR – [86.8] 8.6:38 139 365 55a 2.9 184:50 [98.4] 0.33:43 87 105 56a 0.73 272:40 936:33 4.71:36 26 27 56b 1.4 5.33:93 16.4:74 0.95:33 13 1.5 56c 1.1 45.1:50 27.2:65 0.87:37 4 5 56d 1.9 149:31 [56.9] 3.3:74 349 25 57b 2.1 95:53 99.6:36 1.2:89 23 4.5 57c 6.5 5.2:27 [9.5] 1.4:72 6 2.3 57d 4.2 a Binding assays using [3H]Cl-DPDPE (DOR), [3H]U69,593 (KOR) and [3H]DAMGO (MOR) in recombinant human opioid receptors transfected into Chinese hamster ovary cells b % stimulation of [35S]GTPgS binding relative to, or antagonism of, the standard agonists DAMGO (MOR), DPDPE (DOR) and U69593 (KOR)

N NHR

O HO

N R'

59a R = CH2CH3, R' = H 59b R = (CH2)2CH3, R' = H 59c R = CHO, R' = H 59d R = CHO, R' = (CH2)2F 59e R = CHO, R' = (CH2)3F

Fig. 13 17-Cyclopropylmethylnoraminomorphindole derivatives

reduced tolerance and dependence. In the functional assay the 14-acylaminomorphindoles (57) had higher DOR efficacy than the equivalent alkylaminomorphindoles (56) but similar potencies. The phenylacetylaminomorphindole (57c) had high DOR efficacy and potency and also good selectivity in potency and efficacy over the KOR and MOR partial agonism; thus it was a potent, selective DOR agonist. The SAR for the phenylalkylaminomorphindoles (56c,d) and phenacylaminomorphindoles (57c,d) showed that MOR efficacy was greater for the derivatives with the shorter two carbon C14 side chain. In this respect it followed the pattern for the phenylalkylamino- and phenacylamino-N-cyclopropylmethyldihydronormorphinones where only the side chain derivatives with two carbon atom chains had any MOR agonist effects in vitro [18]. Opioid receptor binding data have been reported for a series of 17-cyclopropylmethylnoraminomorphindole derivatives (59, Fig. 13) [56]. All but the propylamino derivative (59b) had subnanomolar affinity for DOR which reduced its DOR selectivity compared with ethylamino- (59a) which was the only new ligand comparable in selectivity with naltrindole (58b). The indolic-substituted formylamino- derivatives were investigated as potential PET ligands for imaging studies of DOR. Alkylation of the indolic N-atom allowed retention of high affinity for DOR. The fluoroethyl substituent (59d) had little effect on selectivity but fluoropropyl (59e) lowered selectivity.

116


5 Conclusions The groundbreaking work of Professor Gordon Kirby and his collaborators which provided viable synthesis of 14-aminocodeinone and 14-amino-7,8-dihydrocodeinone allowed the preparation of a wide range of derivatives. Among these the 14-cinnamoylaminodihydromorphinones have been most studied as a result of their unrivalled ability to provide MOR-selective irreversible antagonism in clocinnamox (C-CAM) and methcinnamox (M-CAM). The codeinone, methoclocinnamox (MC-CAM), related to C-CAM is a MOR partial agonist and long-term pseudoirreversible MOR antagonist which was considered as a follow-up to buprenorphine as a treatment for opiate abuse and depression. The 14-cinnamoyl derivatives of naltrexone, equivalent to C-CAM and M-CAM, had shorter duration of action and were less effective as pseudoirreversible antagonists. The 14-O-cinnamoyldihydrocodeinone derivative related to MC-CAM, in vivo had no MOR agonist activity and no pseudoirreversible MOR antagonist activity.

References 1. Viscusi ER, Gan TJ, Leslie JB, Foss JF, Talon MD, Du W, Owens G (2009) Peripherally acting m-opioid receptor antagonists and postoperative ileus: mechanisms of action and clinical applicability. Anesth Analg 108:1811–1822 2. Buckett WR (1964) The relationship between analgesic activity, acute toxicity and chemical structure in esters of 14-hydroxycodeinone. J Pharm Pharmacol 16:68T–71T 3. Buckett WR (1965) Some pharmacological studies with 14-(cinnamoyloxy)codeinone. J Pharm Pharmacol 17:759–760 4. Buckett WR, Bosman HH (1972) New morphinone derivatives and their preparation. UK Patent 1 300 419 5. Bentley KW, Horsewood P, Kirby GW, Singh S (1969) Diels-Alder adducts from thebaine and nitroso-arenes. J Chem Soc D:1411 6. Schwab L (1980) 14-(Arylhydroxyamino)codeinones and derivatives as analgetics and antagonists. J Med Chem 23:698–702 7. Allen RM, Kirby GW, McDougall DJ (1981) The nitration of thebaine with tetranitromethane. J Chem Soc Perkin Trans I 26:1143–1147 8. Kobylecki RJ, Guest IG, Lewis JW, Kirby GW (1980) 14-Amino derivatives of morphine, methods of making them and analgesic compositions containing them. US Patent 4241066 9. Kobylecki RJ, Guest IG, Lewis JW, Kirby GW (1980) 14-Amino derivatives of morphine, methods of making them and analgesic compositions containing them. US Patent 4241067 10. Horsewood P, Kirby GW (1980) Preparation of 14-beta-hydroxyamino and 14-beta-aminocodeinone from thebaine. J Chem Res (S) 401:(M)4880 11. Archer S, Seyed-Mozaffari A, Osei-Gyimah P, Bidlack JM, Abood LG (1983) 14b-(2-Bromoacetamido)morphine and 14b-(2-bromoacetamido)morphinone. J Med Chem 26:1775–1777 12. Kirby GW, Sweeney JG (1973) Nitrosocarbonyl compounds as intermediates in the oxidative cleavage of hydroxamic acids. J Chem Soc Chem Commun:704–705 13. Kirby GW, McLean D (1985) An efficient synthesis of 14b-aminocodeinone from thebaine. J Chem Soc Perkin Trans I 1443–1445 14. Lewis J, Smith C, McCarthy P, Walter D, Kobylecki R, Myers M, Haynes A, Lewis C, Waltham K (1988) New 14-aminomorphinones and codeinones. NIDA Res Monogr 90:136–143


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Top Curr Chem (2011) 299: 121–140 DOI: 10.1007/128_2010_83 # Springer-Verlag Berlin Heidelberg 2010 Published online: 31 July 2010

Nonpeptidic Delta (d) Opioid Agonists and Antagonists of the Diarylmethylpiperazine Class: What Have We Learned? Silvia N. Calderon

Abstract The discovery of the selective delta (d) opioid agonists SNC 80 and BW373U86, which possess a diarylmethylpiperazine structure unique among opioids, represented a major advance in the field of d-opioid ligands. Extensive research has recently been performed to uncover the structure–activity relationships (SAR) of this class of ligands, thereby providing valuable tools for the pharmacological characterization of the d opioid receptor. This review focuses on the SAR of this unique series of ligands, and provides an overview of the various chemical routes that have been developed and optimized through the years to allow the syntheses of these ligands on a multigram scale. The search for selective d opioid agonists and antagonists, as well as for those with mixed opioid agonist properties with potential therapeutic value, continues. Several questions regarding the interaction at the molecular level of diphenylmethylpiperazine derivatives and related analogs with opioid receptors and in particular with the d opioid system still remain unanswered. Indeed, the development and pharmacological characterization of novel nonpeptidic d opioid ligands remains an active area of research, as it may provide a better understanding of the role of this receptor in multiple disease states and disorders. Keywords d Agonists d Antagonists Diphenylmethylpiperazines Nonpeptidic delta ligands SNC 80

S.N. Calderon Center for Drug Evaluation and Research, Food and Drug Administration, 10903 New Hampshire Ave, Silver Spring, MD 20993, USA e-mail: [email protected] Opinions and assertions contained herein are those of the author, and are not to be construed as representing those of the Food and Drug Administration.

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Contents 1 2

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Structure–Activity Relationship of the Selective Nonpeptidic d Opioid Agonist SNC 80 and Related Derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Diphenylmethylpiperazines and Diarylmethylpiperazines . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Diarylalkenyl Derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 4-Diarylaminopiperidines and 4-Diarylaminotropanes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4 Diphenylmethyl Bridged Piperazine Derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Nonpeptidic d Opioid Antagonists of the Diarylmethylpiperazine Type . . . . . . . . . . . . . . . . . 4 Synthesis of SNC 80 and Related Diphenylarylpiperazines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

122 124 124 129 131 132 133 133 137 138

Abbreviations Boc Bu Cbz ee Et Me MeOH Ph t-Bu THF TMEDA

tert-Butoxycarbonyl Butyl Benzyloxycarbonyl Enantiomer excess Ethyl Methyl Methanol Phenyl tert-Butyl Tetrahydrofuran N,N,N0 ,N0 -Tetramethyl-1,2-ethylenediamine

1 Introduction Intensive research over the last 2 decades has provided a better understanding of the structure, distribution, and pharmacology of the opioid receptors. Three types of opioid receptors known as mu (m), delta (d), and kappa (k) receptor types have been identified and the mRNA encoding these receptors has been isolated. It has become clear that each receptor mediates unique pharmacological responses and is differentially distributed in the central nervous system [1–6]. Thus, the design and synthesis of selective ligands that interact with each of these receptors, as well as the characterization of the pharmacological effects that these ligands mediate, remain a very active area of research. The d opioid receptor has become an attractive target for the development of new drugs with potential therapeutic

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benefits in analgesia, addiction, affective disorders, and neurodegenerative and gastrointestinal disorders [7–14]. Work from several research groups indicates that agonists acting at the d opioid receptor produce analgesia in animal models of pain while appearing to lack the undesirable effects associated with prototypic m opioid agonist such as morphine [15, 16]. Morphine-like drugs produce analgesia and multiple side effects such as respiratory depression, constipation, miosis, and physical dependence. Three families of endogenous opioid peptides known as b-endorphins, enkephalins, and dynorphins, have been identified. Although most of these endogenous opioids have little selectivity for opioid receptors, in general it is accepted that b-endorphins, enkephalins, and dynorphins displayed greater affinity for m, d, and k receptors respectively [17]. The discovery of endogenous opioid peptide agonists with potent opiate agonist activity at the d receptor was followed by the synthesis and characterization of many other selective peptidic ligands [18]. However, peptidic ligands are poorly absorbed orally, sensitive to degradation by peptidases, and generally do not cross the blood brain barrier. Thus, to characterize better the pharmacological profile of the d opioid receptor it was imperative to develop highly selective and potent opioid ligands with good central bioavailability [19]. The understanding of the structure and function of receptors is largely dependent on the discovery of agonist and antagonists. Antagonists have proven to be invaluable tools in pharmacology because they allow the identification of key physiological functions mediated by the receptors through the blockade of their biological responses. The development and pharmacological characterization of the first nonpeptidic d opioid selective antagonist, naltrindole (1) (Figs. 1 and 2) by Portoghese in 1988 [20] provided the framework for research aimed at understanding the functions mediated by the d opioids receptors. Naltrindole was designed based on the message–address concept previously introduced by Schwyzer [21], by which a naltrexone-derived recognition unit envisioned as the message component was attached to a benzene moiety that simulated the phenylalanine and considered the address unit of the enkephalins. Another major advance in this area was the discovery of BW373U86 ()-2 (Fig. 1) [22, 23], a selective nonpeptidic d opioid receptor agonist with a new carbon–nitrogen skeleton, followed by the identification and characterization of the highly d selective SNC 80 (þ)-3 (Fig. 1) [24]. Unlike naltrindole, which has a classical opioid ligand structure, BW373U86 and SNC 80 have a chemical structure not commonly associated with opiates. Further research led to the characterization of other nonpeptidic agonists and antagonists that belong to other chemical classes. The agonist group includes morphinan derivatives such as SIOM (4) (Fig. 1) [25], isoquinoline derivatives like TAN-67 (5) (Fig. 1) [26, 27], and the pyrrolooctahydroisoquinolines derivatives. The latter chemical class served as a template for the development of agonists such as SB-219825 (6) (Fig. 1) and the antagonists SB-205588 (7) and SB-220718 (8) (Fig. 2) [28–30]. So far, only one antagonist, DPI2505 (9) (Fig. 2) [31], has been identified within the diarylmethylpiperazine series. A series of selective d opioid antagonists with a biarylpiperidine structure has recently been reported. Compounds 10 and 11 (Fig. 2) exemplify some of the antagonists identified in these series [32].

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S.N. Calderon O Et2N

N

OH

OH CH3

N O

HO

CH3

N H

N

BW373U86 (±)-2

Naltrindole (1) O Et2N

N OH

H OCH3 N

CH3 HO

CH3

O

SIOM (4)

SNC80 (+)-3

CH3

O

N

CH3

H

H

O

EtN

N N

OH TAN-67 (5)

N H

NEt2

OH SB-219825 (6)

Fig. 1 Structures of the delta opioid antagonist Naltrindole and of prototypic nonpeptidic delta opioid agonists

This review focuses on the advances on the structure–activity relationship (SAR) studies on the diarylmethylpiperazine derivatives of the SNC 80 type, with agonist or antagonist properties at the d opioid receptors, as well as provides a description of the synthetic procedures used in the synthesis of SNC 80 and its analogs.

2 Structure–Activity Relationship of the Selective Nonpeptidic d Opioid Agonist SNC 80 and Related Derivatives 2.1

Diphenylmethylpiperazines and Diarylmethylpiperazines

As previously captured in other related reviews of the topic [28, 29, 33], the discovery and identification of BW373U86 ()-2 [22, 23] with a unique carbon


125 H

N

EtN

OH

N H HO

O

N H

OH

Naltrindole (1)

SB-205588 (7) O

N

Et2N OH

OH N

CH3 HO

O

N H

NEt2

CH3

SB-220718 (8) Et

N

CH3

O

DPI2505 (9)

OH O

Et NH2

Et2N

O

OH

N N

(10)

(11)

Fig. 2 Structures of nonpeptidic delta antagonists

skeleton, unrelated to other d agonists, was the starting point of extensive research in the area of nonpeptidic d ligands. The SAR studies and characterization of ()-2 derivatives was reported in the patent literature [34]. Although BW373U86 showed a high degree of selectivity for d opioid receptors some of its effects proved to be mediated through m opioid receptors [34, 35]. In an effort to characterize further the structural ligand requirements of the d opioid receptor, the synthesis and assignment of the absolute configuration of the optical pure enantiomers of BW373U86, its benzylic epimers, and their methyl ethers was later reported [24]. The affinities of these enantiomers and immediate synthetic precursors for m opioid receptors and d opioid receptors were determined by inhibition of binding of [3H]DAMGO to rat brain membranes and [3H]-DADLE to mouse brain membranes depleted of m binding sites by the pretreatment with the irreversible ligand benzimidazole isothiocyanate (BIT), respectively. In addition, the opioid activity of

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these compounds was next evaluated in the isolated mouse vas deferens (MVD) and in guinea pig ileum (GPI) bioassays. From this first series of compounds the (þ)-methoxy precursor of (þ)-2, known as SNC 80 (þ)-3, displayed a remarkable m/d selectivity in both receptor binding and bioassays and showed opioid agonist activity in the mouse, exerting its effects through both d1 and d2 receptors [36]. To understand further the interaction of this distinctive diphenylpiperazine structure with the d opioid receptor, the effect of the replacement of the methoxy group present in SNC 80 template by a hydroxyl function, hydrogen, fluorine, or iodine was evaluated [37]. The pharmacological profile of the most selective enantiomeric precursors of BW373U86 as well as other SNC 80 derivatives is presented in Table 1. These first SAR studies showed that compounds which retained the same spatial orientation at the benzhydryl (a) position as that of SNC 80 displayed higher selectivity than their benzylic epimers. In addition, the replacement of the methoxy group by either hydrogen, (þ)-13, or fluorine, (þ)-14, resulted in highly selective analogs of SNC 80. Analysis of the effect of a positional change of the N,N-diethylamide function as well as the effect of diverse alkyl substitution at this function followed. A series of N-monoalkyl- and dialkylbenzamide derivatives of SNC 80 were synthesized and their pharmacological profile reported [38]. Regarding the binding affinity of the amide positional isomers of the desmethoxy derivative of SNC 80, for d opioid receptors, both the meta and the ortho diethylbenzamides displayed lower affinity than the para counterpart. The affinity of the ortho derivative was in the mM range whereas the meta derivatives still retain nM affinity for the d opioid receptor. Compared to the para benzamide analog, the affinity of the meta derivative was 90-fold lower [38]. Alkyl substitution at the amide group had a more intense effect in the SNC 80 series, N-monoalkylbenzamide derivatives showed lower affinity than the N,N-dialkylbenzamide derivatives, and the N-unsubstituted benzamide had the lowest affinity for the d opioid receptors. The N-monoalkylbenzamides also showed poor selectivity, while the N,N-dialkyl derivatives showed a good m/d ratio. From this series, two compounds, the dimethyl substituted, (þ)-15, and the methyl-ethyl substituted benzamides, (þ)-16, displayed high affinity and selectivity for d opioid receptors and proved to be d agonists in MVD preparations (Table 1). In the (þ)-2 series, the meta diethyl amide derivative retained high affinity for d as well as for m opioid receptors. Movement of the diethyl amide from the para to the meta position resulted in less d selective ligands with a mixed d/m agonist profile in these series. Changing the meta-diethylamide to a meta-N-methylanilide resulted in potent mixed d and m agonists [39]. Zwitterionic analogs of SNC 80 designed by replacement of the diethylamide benzamide ring by various 7-yl-3,4-dihydro-2(1H)-isoquinoline-carboxylic acids have recently been described in the literature as selective peripheral d opioid receptor agonists [40]. Analysis of the need of the dimethyl substituted piperazine as well as the role of the two basic nitrogens (N-1 and N-4) and the influence of substituents on N-4 followed [41]. Initially it was noted that the trans-dimethyl groups on the piperazine ring were not critical for recognition at the d receptor [41–43]. Since it was

N-1

CH3

(S)

N

(R)

N

(R)

H CH3 N-4

R

Compound

–R; –R1

Binding (IC50 nM SD) Agonist activity (IC50 nM SEM) dc IC50 ratio m/d MVD (d) IC50 ratio GPI/MVD mb ()-2, BW373U86 –OH; Et2N– 46.3 4.42 0.92 0.06d 50.3 0.2 0.02 715 (+)-3, SNC 80 (aR, 2S, 5R) –OCH3; Et2N– 2,467 200 2.88 0.35 857 2.73 0.50 1,996 9,138 823 4.88 0.52 1,873 30.9 4.0 49 ()-12 (aR, 2R, 5S) –OCH3; Et2N– (+)-13 (aS, 2S, 5R)a –H; Et2N– >2,500 0.94 0.09 >2,660 10.56 0.38 813 1,977 260 2.92 0.36 677 9.77 1.28 265 (+)-14 (aR, 2S, 5R) –F; Et2N– (+)-15 (aR, 2S, 5R) –OCH3; Me2N– 3,261 719 4.8 0.69e 679 8.92 837 7,373 558 4.2 0.47e 1,755 4.45 958 (+)-16 (aR, 2S, 5R) –OCH3; Et(Me)N– a The spatial orientation at the a position of (þ)-13 is the same as in (þ)-3 and the designation of aS-(þ)-13 is arbitrary and results from the application of the Cahn, Ingold, and Prelog nomenclature rules b Binding against [3H]-DAMGO in rat brain membranes [24, 37, 38] c Binding against [3H]-DADLE to rat brain membranes using DAMGO to block m receptors [37] d Binding inhibition of [3H]-DADLE in mouse brain membranes [24] e Binding of [3H]-DADAL in rat brain membranes [38]

R1

O

Table 1 Receptor affinity and agonist activity of selective diphenylmethylpiperazines [24, 37, 38]

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determined that the lack of dimethyl substitution at the piperazine level did not affect the selectivity or affinity of these type of ligands for the d receptor, and it was known that the desmethoxy derivative of SNC 80, (þ)-13, retained high affinity and selectivity for the same receptor, a simplified 4-(1-piperazinylbenzhydryl)-N, N-diethylbenzamide skeleton was selected as a template to analyze the influence of the N-4 substitution modifications in the affinity and selectivity for the d receptor. It was shown that the replacement of the N-allyl group with an alkyl moiety reduced the affinity at the d receptor; however, the benzyl derivative retained high affinity and a good agonist profile [41, 44]. The replacement of the N-4 nitrogen with its allyl group by oxygen or a methylene group to give the corresponding morpholino or piperidine analogs, respectively, abolished binding to opioid receptors, indicating that the basic nitrogen N-4 is an important structural element in this series required for the interaction with the d receptor. In contrast, replacement of the N-1 by a methylene group did not have such a dramatic effect, showing that N-1 could be modified, retaining moderate affinity for the d receptor. It was also indicated that a cyclic amine was required for recognition at the receptor since diarylmethyl diamines showed low affinity at the d receptor and did not bind at other opioid receptors. It was then concluded that N-1 was relatively unnecessary for binding to the d receptor. Later, using the SNC 80 structure as a template, the effect of the N-4 substitution on the affinity and activity of these ligands was studied in more detail. It was found that the 4-crotyl, 2-methylallyl, and benzyl compounds displayed high selectivity and proved to be as efficacious agonists as SNC 80 in the [35S]GTPgS function assay. It was also shown that N-cyclopropylmethyl and N-allyl substitution in the SNC 80 series did not confer antagonist properties to these ligands, unlike what it is observed in the morphinan series at the m opioid receptors [45]. The identification and pharmacological evaluation of an SNC 80 simplified structure gave origin to a new series of d opioid agonists [46]. Initially the structure of SNC 80 was modified to reduce the number of chiral centers to an analog in which the piperazine methyl groups and the N-allyl group were eliminated, and in which the aryl methoxy group was replaced by hydrogen (17) (Table 2). This template retained strong d binding affinity, high selectivity, and showed full agonist properties [46]. Substitution of the phenyl ring with a variety of substituted aryl and heteroaryl groups followed. The replacement of the phenyl ring with a cyclohexyl ring (18) gave a sharp decrease in affinity and selectivity when compared to the parent compound 17 (Table 2), whereas compounds with a 1-naphthyl or 2naphthyl substitution had improved binding affinity and agonist potency. In this series the 8-quinolyl derivative (19) and the 2,2-dimethyl-2,3-dihydro-1-benzofuran-7-yl derivative (20) had significantly high binding affinity and improved selectivity. It is of particular interest that another d opioid selective nonpeptidic piperazine derivative agonist, SL-3111 (1-(4-tert-butyl-30 -hydroxy)benzhydryl-4-benzylpiperazine), was designed and synthesized based on the identification of the essential pharmacophores for peptide ligands to interact with d receptors, and on a model of the proposed bioactive conformation of the potent, conformationally constrained d-opioid peptide ligand [(2S,3R)-TMT1]DPDPE [47]. This new series used the


129

Table 2 Receptor affinity and agonist activity of simplified SNC 80 derivatives with a diarylmethylpiperazine structure [46] O Et2N

H

R

N N H

Compound –R

Binding (IC50 nM SD) m db IC50 ratio m/d 8,150 370 11.3 2.3 740 185 14 98.4 11.0 1.9 632 142 0.51 0.05 1,239 785 82 1.15 0.12 682 a

Agonist activityc Emax (EC50 nM SEM) (%)

–Phenyl 36.5 5.9 104 5 –Cyclohexyl ND ND –8-Quinolyl 3.60 0.24 104 2 –2,2-Dimethyl-2,37.10 1.05 101 4 dihydro-1benzofuran-7-yl a Binding inhibition of [125I]-FK33824 in cloned m receptors [45] b Binding against [125I]-[D-Ala2] deltorphin II in cloned d receptors [46] c Agonist potency (EC50) measured in [35S]GTPgS binding assays in membranes expressing human d receptors [46] 17 18 19 20

piperazine template to incorporate relevant pharmacophore groups such as a phenol ring, phenyl groups, and a hydrophobic moiety. The hydrophobic group was design to mimic the hydrophobic character of the D-Pen residues in DPDPE. Racemic SL-3111 showed high affinity and selectivity for d opioid receptors. Binding studies on the cloned wild-type and mutated human d opioid receptors suggested that the new nonpeptidic ligand has a binding profile similar to that of DPDPE but different from that of SNC 80 [47]. In this series, the replacement of N-4 by a methylene group have a dramatic effect on the binding to opioids receptors. Similarly to what it was observed in the SNC-80 series, N-4 is essential for binding at the opioids receptors [48].

2.2

Diarylalkenyl Derivatives

The simplified SNC 80 structure 17 was further modified by replacement of the N-1 of the piperazine with carbon double bond, leading to a new series of diarylalkenylpiperidines [49]. These new compounds were evaluated for their affinity for all three human opioid receptors and their agonist activity was determined using the

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[35S]GTPgS binding assays. These new series of compounds resulted in the most selective d ligands so far described and from the chemistry point of view they have the advantages of not having chiral centers and being easily synthesized. The simplified structure 21 shows higher affinity for d receptors in binding studies and higher selectivity than 17 and SNC 80 under the same conditions of the study (Table 3). In this series it was also shown that the aryl ring could accommodate a 3-OMe group (22) or a 3-F substituent (23) without affecting the selectivity and affinity for the d receptor. The position of the substituent on the phenyl ring had a greater influence in both the binding affinity and agonist activity. It was observed that substitution at the para-position of the phenyl ring as well as di-ortho-substitution appeared to be detrimental. As was observed in the diarylmethylpiperazine series [46], the replacement of the phenyl ring by other aryl or heteroaryl groups did not affect affinity and selectivity. Unlike what was observed in the SNC 80 series, N-allyl substitution (24 and 25) resulted in compounds with lower affinity than the unsubstituted parent compound 21. Table 3 Receptor affinity and agonist activity of N,N-diethyl-4-(phenylpiperidin-4-ylidenemethyl)benzamide [49] O Et2N R

N R1

Compound –R; –R1

Binding (IC50 nM SD) Agonist activityc b m d IC50 Emax (EC50 nM (%) ratio m/d SEM) 320 58 1.31 0.17 245 3.67 0.70 100d a

(+)-3, SNC80 17 21 22 23 24 25

–

– 8,150 370 11.3 2.3 740 36.5 5.9 104 5 –Phenyl; –H 3,800 172 0.87 0.23 4,370 7.2 0.9 110 3 –3-MeO-Phenyl;–H 5,250 445 1.56 0.26 3,370 13.7 3.2 109 7 –3-F-Phenyl; –H 3,570 444 0.74 0.14 4,820 4.6 0.5 112 6 –Phenyl; –Allyl 3,330 275 3.00 0.69 1,110 28.3 2.1 98 3 –Phenyl; –3,31,030 100 2.09 0.11 493 14.1 1.8 106 3 Dimethyl allyl a Binding inhibition of [125I]-FK33824 in cloned m receptors [49] b Binding against [125I]-[D-Ala2] deltorphin II in cloned d receptors [49] c Agonist potency (EC50) measured in [35S]GTP[gS] binding assays in membranes expressing human d receptors d Reference compound [49]


2.3

131

4-Diarylaminopiperidines and 4-Diarylaminotropanes

Transposition of the piperazyl N-1 and the benzylic carbon of SNC 80 gave origin to a new series of derivatives known as 4-diarylaminopiperidines. The synthesis of the first analogs with this structure was reported in the patent literature [50]. In this series, replacement of the aryl ring that bears the –OCH3 group in SNC 80 with other aromatic rings or with substituted phenyl rings as well as piperazine ring contraction was explored, but no biological information was provided. Later, the racemic cis- and trans-4-[N-allyl-3-methyl-4 piperidinyl) phenylamino]-N,N-diethylbenzamides 26 and 27 [51] were synthesized. The cis racemate was found to have good affinity and selectivity for the d receptor in radioligand binding assays (Table 4). In addition, the cis racemate was found to stimulate [35S]GTPgS binding at the d receptor with no measurable stimulation of either the m or k receptors. Optical resolution of 26 revealed that the ()-26 isomer was responsible for the d opioid activity of the racemate and behaved as a full agonist. Further information on SAR studies on compounds of the general structure 26 has been reported [52, 53]. These studies have shown that in general these 4-diarylaminopiperidines are less selective relative to their piperazine analogs due to lower d affinity and an increased affinity for m receptors. Data available from the N-substitution indicates that, in accord with the findings reported in the piperazine series [41, 45], electron rich substituents provide compounds of higher affinity and selectivity (29, 30) for the d opioid receptor. It was also noted that unsubstituted or hydroxyl aryl derivatives in this series were less selective than their methoxy counterparts, possessing greater affinity for both m and d receptors (27, 28). In these series the N-(4-diethylcarboxamidophenyl) group was found to be essential for the interaction at the d receptor. In addition the cis relative stereochemistry between the 3- and 4-substituents in the piperidine ring, a trans-crotyl or allyl substituent on the basic nitrogen, and either no substitution or hydroxyl substitution in the aryl ring were features that enhanced the affinity of these ligands for d receptor receptors. In contrast, binding to the m receptor appeared to be enhanced by a hydroxyl substitution in the aryl ring, the presence of an additional methyl group in the 2-position of the piperidine ring in a relative cis relationship to the 4-amino group, and by N-substitution with groups such as cyclopropylmethyl. In a different series, the effect of replacing the piperidine by a tropane ring was explored [54]. This new class of 4-diarylaminotropanes exhibited high affinity for d receptors with Kis in the nanomolar range. Affinity was measured by inhibition of [3H]-DPDPE and [3H]-DAMGO to d opioid and m opioid receptors from rat brain membranes, respectively. The endo isomers were in general more potent and selective than their nonbridged 4-diarylaminopiperidine counterparts and also more selective that the exo isomers. The SAR of the endo isomers parallels other series [41, 45, 51, 52].

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Table 4 Receptor affinity and agonist activity of 4-[(N-substituted-4-piperidinyl)arylamino]-N,Ndiethylbenzamides [53] O Et2N N

R2 CH3

N R1

Binding (IC50 nM SD) ma db

Compound

–R1; –R2

(+)-3, SNC 80 ()-26 cis (+)-26 cis, (3R,4S) ()-26 cis, (3S,4R) ()-27 trans

– –Allyl; –H –Allyl; –H

Agonist activityc IC50 ratio (ED50 nM Emax m/d SEM) (%) 1,614 131 1.57 0.19 1,030 317 54 142 6 1,212 132 11.9 0.9 102 3,500 500 63 5 – >4,854 139 11.4 >35 NSd

–Allyl; –H

2,623 307 5.58 0.31 470

2,508 248

126 5

–Allyl; – 4,872 541 379 23 13 – – OCH3 ()-28 cis –Allyl; –OH 126 9 1.85 0.24 68 90.6 32.4 98 7 ()-29 cis trans-Crotyl; 3,590 285 7.0 0.6 510 – – –H ()-29 cis trans-Crotyl; 938 71 2.72 0.62 345 1,129 230 121 7 –H ()-30 cis trans-Crotyl; 40.8 7.8 2.00 0.12 20 28.6 12.5 121 9 –OH a Binding inhibition of [3H]-DAMGO in rat brain membranes [53] b Binding against [3H]-[DADLE] in rat brain membranes [53] c Agonist potency, ED50 and Emax, measured in [35S]GTP[gS] binding assays in guinea pig caudate membranes d No simulation [53]

2.4

Diphenylmethyl Bridged Piperazine Derivatives

The replacement of the piperazine ring of SNC 80 with a 6,8-diazabicyclo[3.2.2] nonane scaffold with its trimethylene bridge corresponding to the methyl piperazine substituents in SNC80 gave a series of compounds with high affinity for the d opioid receptors [55], though relatively lower when compared to SNC80. The analog with the same spatial distribution at the benzhydryl position as SNC80 displayed higher affinity than its diastereomer. Relative to SNC80 these compounds had lower affinity than SNC80.


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3 Nonpeptidic d Opioid Antagonists of the Diarylmethylpiperazine Type As stated above, alterations in the N-substituent to those which tend to confer m antagonism in the morphinan series (e.g., cyclopropylmethyl) resulted in d agonists [44]. Indeed, d antagonists have proved quite elusive in the diarylmethylpiperazine series, and the only antagonist reported to date with a structure of the SNC 80 type is DPI2505 (9) (Fig. 2) [31]. The shift of a methyl group in the piperazine ring from the 2-position to the 3-position in the piperazine ring in conjunction with one carbon homologation of the N-4-allyl substituent dramatically changed the pharmacological profile of the compounds in these series. More recently, a series of novel 4,4-diphenylpiperidine and 3,3-diphenyl piperidine derivatives has been characterized as selective antagonists [32]. Compounds 10 and 11 (Fig. 2) are examples of selective antagonists identified in these series. These compounds expand on the SAR of the simplified piperidine analogs of SNC 80 and BW373U86. In these series, the effect of modifications at the amide group, as well as the effect of substitution of the meta-phenol group present in BW373U86 by other groups such as low molecular amides, and the effect of the substitution at the piperidine nitrogen were evaluated. In both series, alkyl group modifications were broadly tolerated, and the replacement of the amide with tertiary carbinols was also well tolerated. Primary carboxamides were well tolerated at the meta-position, giving compounds with high affinity and selectivity for the d opioid receptor. Regarding the substitution at the piperidine nitrogen, within the 4,4-biaryl piperidine, long aliphatic and branched alkyl chains resulted in good and selective antagonists. Within the 3,3-diphenyl series, small and medium size alkyl groups at the piperazine nitrogen seems to confer good selectivity and antagonist properties [32].

4 Synthesis of SNC 80 and Related Diphenylarylpiperazines Several strategies currently exist for the preparation of diphenylmethylpiperazines in optically pure form [24, 44, 56]. SNC 80 and its analogs were initially synthesized by displacing the appropriate benzhydryl chloride with the corresponding enantiomers of the trans-1-allyl-2,5-dimethylpiperazine [24]. This initial approach offered the dual advantage of requiring one optical resolution, that of the trans-1allyl-2,5-dimethylpiperazine, and that both optically pure, readily separable, benzylic epimers were obtained in a single step. Though this synthetic route seemed to be adequate to evaluate the effect of the stereochemistry on the d receptor selectivity, a more efficient stereoselective synthesis of the diarylmethylpiperazine type derivatives that allowed the synthesis of larger quantities of these derivatives was developed by Bishop and McNutt [56]. This synthetic approach involves the stereoselective addition of an organometallic nucleophile to a “masked iminium”

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salt to avoid the isolation of the desired iminium salt. The a-benzotriazolylamine was chosen, due to its ability to undergo nucleophilic displacement of the benzotriazole [57, 58]. This adaptation of the Katritzky method for forming tertiary amines provided high yields of the desired diarymethylpiperazine diastereomers with a high diastereomeric excess. Following this approach, compound (þ)-2 was synthesized with an 86% diastereomeric excess and 60% yield. Compound (þ)-2 was synthesized from the 4-N,N-diethyl-4-formylbenzamide, benzotriazole and the ()-(2R,5S)-1-allyl-2,5-dimethylpiperazine (Scheme 1). These reagents were heated under reflux in toluene with a Dean–Stark trap attached to remove water. The intermediate benzotriazole was not isolated, and the entire toluene solution was added at room temperature directly to a solution of 3-(tert-butyldimethylsilyloxy) phenylmagnesium bromide in THF at room temperature. Subsequent acidic removal of the silyl protecting group gave (þ)-2 in 60% yield in a 93:7 ratio, in favor of the desired diastereomer [56]. The third approach to obtain diarylmethylpiperazine derivatives uses the highly stereospecific chiral oxazaborolidine-catalyzed reduction, using catecholborane as the reductant of the 4-bromobenzophenone chromium tricarbonyl complex, as described by Corey and Helal [59], followed by the stereospecific displacement of the hydroxyl benzyl group by the N-substituted-piperazine [44]. As outlined in Scheme 2, Delorme et al. [44] used this approach for the enantioselective synthesis of compound 31, (þ)-4-[ (aS)-a-(4-benzyl-1-piperazinyl)benzyl]-N,N-diethylbenzamide. Lithiation of the readily available benzene chromium tricarbonyl with n-BuLi in the presence of TMEDA in THF at 78 C, followed by addition of O H

Et2N

N N N

H

H N

H N N N

O

+

N

a

+ N

N O

NEt2 b, c O Et2N

H OH N N (+)-2

Scheme 1 Reagents. (a) Toluene, 111oC, (-H2O); (b) 3(3-tert-Butylmethylsilyloxy)phenylmagnesium bromide, THF, 25 oC; (c) 3N HCl [56]

Nonpeptidic Delta (d) Opioid Agonists and Antagonists Br

135 Br

d

a; b; c Cr(CO)3

H

Cr(CO)3

Cr(CO)3

O

OH Br

Br

e; f

H N

g Cr(CO)3

H N N

N

O h; i; j

Et2N

H N N 31

Scheme 2 Reagents. (a) n-BuLi, TMEDA, THF, -78 oC; (b) CuBr. Me2S, THF, -78 oC; (c) p-BrC6H5COCl (74%); (d) , catecholborane, toluene, -78 oC to - 40oC (91%); (e) HBF4OEt2, CH2Cl2, -60 oC; (f) N-Benzylpiperazine, CH2Cl2 (75%); (g) (NH4)2Ce(NO3)6, MeOH (95%); (h) n-BuLi, CO2, THF, -78 oC; (i) SOCl2, CH2Cl2; (j) Et2NH, CH2Cl2 (34% overall) [44]

copper(I) bromide dimethylsulfide complex (CuBr-Me2S) in THF at 78 C, and by reaction with p-bromobenzoyl chloride in THF at 78 C, gave the desired 4-bromobenzophenone chromium tricarbonyl complex (74% yield). Addition of a solution of the ketone in toluene to the oxazaborolidine catalyst, formed as described by Corey and Halal [59] by refluxing a solution of (S)-()-a,adiphenyl-2-pyrrolidinemethanol in toluene and n-butyl-boronic acid (n-BuB (OH)2), and catecholborane in toluene at 78 C followed by warming of the solution to 40 C, afforded the desired benzhydryl alcohol in 91% ee (91% yield). This alcohol was treated with tetrafluoroboric acid–diethyl ether complex (HBF4. OEt2) in CH2C12 at 60 C followed by the addition of N-benzylpiperazine. Decomplexation using ammonium cerium(IV) nitrate ((NH4)2Ce(NO3)6) in MeOH gave the 4-[(aS)-a-(4-benzyl-1-piperazinyl)benzyl]-bromobenzene derivative (95% yield). Transmetallation in dry THF at 78 C with n-BuLi, followed by bubbling with carbon dioxide, afforded the corresponding acid, which was treated with thionyl chloride followed by the addition of diethylamine to give compound 31 in 91% ee (34% overall yield).

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The synthesis of (þ)-2 and that of SNC 80 requires the enantiopure dimethylpiperazine derivative, ()-(2R,5S)-1-allyl-2,5-dimethylpiperazine. Initially this enantiomer was obtained through a classical resolution of the racemic piperazine. Though practical, this resolution required the use of the unnatural and expensive ()-camphoric acid to obtain the desired piperazine enantiomer. Janetka et al. [60] described a new, nonchromatographic, enantioconvergent, synthetic approach that allows the large scale synthesis of the required piperazine enantiomer. This procedure involves an efficient optical resolution using relatively inexpensive resolving agents, followed by interconversion of the unwanted (þ)-enantiomer to the desired ()-enantiomer. Allylation of the trans-dimethylpiperazine gives a mixture of H N N H trans -2, 5-Dimethyliperazine h

a

H N

H N

N

N

N

N

(–)-enantiomer

(+)-enantiomer

H N N H

Diallyl-trans-2,5-dimethylpiperazine

b Cbz

Cbz

N

N c

g N

N H d

N

f

Cbz

H N

N e

N

N

N

(t-Boc)

(t-Boc)

(t-Boc)

Scheme 3 Reagents. (a) Allyl bromide/Acetone followed by optical resolution; (b) Benzyl chloroformate, NaHCO3 (aq), CHCl3(98%);(c) Pd/C, H2O, AcOH (100%); (d) di-tert-butyl dicarbonate, CH2Cl2 (98%); (e) Pd/C, Ammonium formate, MeOH (97%); (f) Allyl bromide, K2CO3, Acetone (98%); (g) TFA, Triethylsilane, CH2Cl2(89%); (h) Pd/C, H2O, AcOH [60]


137

unchanged piperazine, ()-trans-2,5-dimethylpiperazine, and 1,4-diallyl-trans2,5-dimethylpiperazine (Scheme 3) [24]. The unchanged piperazine was removed through the hydrobromide salt. The mixture of bases from the filtrate was then treated with succinic acid to give the disuccinate salt of the ()-monoallylpiperazine. The (þ)-enantiomer was removed as its salt with the readily available (þ)-camphoric acid and the mixed bases then resolved with (þ)-tartaric acid to provide about 40% (of the theoretical 50%) of the desired ()-enantiomer of >99% optical purity. This procedure proved to be reproducible and satisfactory, using amounts up to 300 g of the ()-trans-2,5-dimethylpiperazine [60]. To increase the efficiency of the resolution a procedure to convert the undesired (þ)-enantiomer to the desired one was developed. The goal was to interconvert the (þ)-enantiomer to the ()-enantiomer by moving the allyl group from the N-1 to the N-4 position of the undesired enantiomer This was accomplished by the differential protection of the piperazine’s nitrogens as carbamates. To this end, the secondary amine of the (þ)-enantiomer was protected as a benzylcarbamate, followed by catalytic N-deallylation [61] by one-pot palladium-mediated isomerization and hydrolysis of the resultant enamine. Carbamylation of the benzylcarbamate amine with dibutyl carbamate gave the di-tert-butylcarbonate derivative [(2R,5S)-1-benzyloxycarbonyl-4tert-butoxycarbonyl-2,5-dimethylpiperazine]. Removal of the benzyloxycarbonyl protective group, followed by introduction of the allyl group, and removal of the tert-butoxycarbonyl group with trifluoroacetic acid, gave the desired ()-enantiomer in 79% yield from the (þ)-enantiomer (Scheme 3). While the interconversion of enantiomers represented an improvement, the procedure was optimized even further by the recycling of the 1,4-diallyl-2,5-dimethypiperazine, formed in the initial alkylation step, to the trans-2,5-dimethylpiperazine by catalytic deallylation (Scheme 3) [61].

5 Conclusions The discovery of BW373U86 and SNC 80 represented a major development in the search for selective d opioid agonists, and led to a greater understanding of the pharmacology and to the potential role of the d opioid system in therapeutics. Structural manipulation has uncovered a unique SAR for these series of ligands, and led to the identification of selective d opioid agonists and antagonist, as well as of compounds with mixed m and d agonist properties. In the last decade, most of the attention has been given to the identification of selective d opioid ligands. However, research efforts seem to have shifted to the identification of compounds that activate both the d and the m receptor systems because this dual receptor modulation may offer a therapeutic advantage in certain disease conditions such as pain management and analgesia [39]. Regarding the chemistry of the diphenylmethylpiperazines, high yield enantioselective synthetic procedures that allow the multigram synthesis of these compounds have been developed [56, 60].

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44. Delorme D, Berthelette C, Lavoie R, Roberts E (1998) Asymmetric synthesis of diarylmethylamines: preparation of selective opioid delta receptor ligands. Tetrahedron Asymmetry 9:3963–3966 45. Furness SC, Zhang X, Coop A et al (2000) Probes for narcotic receptor-mediated phenomena. 27. Synthesis and pharmacological evaluation of selective delta-opioid receptor agonists from 4-[(aR)-alpha-(2S,5R)-4-substituted-2,5-dimethyl-1-piperazinyl-3-methoxybenzyl]-N, N-diethylbenzamides and their enantiomers. J Med Chem 43:3193–3196 46. Pobleck N, Delorme D, Wei Z-Y et al (2000) New diarylmethylpiperazines as potent and selective nonpeptidic delta opioid receptor agonists with increased in vitro metabolic stability. J Med Chem 43:3878–3894 47. Liao S, Alfaro-Lopez J, Shenderovich MD et al (1998) De novo design, synthesis, and biological activities of high-affinity and selective non-peptide agonists of the delta-opioid receptor. J Med Chem 41:4767–4776 48. Alfaro-Lopez J, Okayama T, Hosohata K et al (1999) Exploring the structure-activity relationships of [1-(4-tert-butyl-3’-hydroxy)benzhydryl-4-benzylpiperazine] (SL-3111), a high-affinity and selective d-opioid receptor nonpeptide agonist ligand. J Med Chem 42:5359–5368 49. Wei Z-Y, Brown W, Takasaki B et al (2000) N,N-Diethyl-4-(phenylpiperidin-4-ylidenemethyl)benzamide: a novel, exceptionally selective, potent delta opioid receptor agonist with oral bioavailability and its analogues. J Med Chem 43:3895–3905 50. Pelcman B, Roberts E (1998) International Patent Application WO 98/28270 51. Thomas JB, Herault XM, Rothman RB et al (1999) (þ/)-4-[(N-Allyl-cis-3-methyl-4piperidinyl)phenylamino]-N,N-diethylbenzamide displays selective binding for the delta opioid receptor. Bioorg Med Chem Lett 9:3053–3056 52. Thomas JB, Atkinson RN, Herault XM et al (1999) Optically pure ()-4-[(N-allyl-3-methyl4-piperidinyl)phenyl-amino]-N,N-diethylbenzamide displays selective binding and full agonist activity for the delta opioid receptor. Bioorg Med Chem Lett 9:3347–3350 53. Thomas JB, Herault XM, Rothman RB et al (2001) Factors influencing agonist potency and selectivity for the opioid delta receptor are revealed in structure-activity relationship studies of the 4-[(N-substituted-4-piperidinyl)arylamino]-N,N-diethylbenzamides. J Med Chem 44:972–987 54. Boyd RE, Carson JR, Codd EE et al (2000) Synthesis and binding affinities of 4-diarylaminotropanes, a new class of delta opioid agonists. Bioorg Med Chem Lett 10:1109–1111 55. Holl R, Jung B, Schepmann D et al (2009) Synthesis and pharmacological evaluation of SNC80 analogues with bridged piperazine ring. ChemMedChem 4:2111–2122 56. Bishop MJ, McNutt RW (1995) An efficient synthesis of the benzhydrylpiperazine delta opioid agonist (þ)-BW373U86. Bioorg Med Chem Lett 5:1311–1314 57. Katritzky AR, Rachwal S, Rachwal B (1987) The chemistry of N-substituted benzotriazoles. Part 4. A novel and versatile method for the mono-N-alkylation of aromatic and heteroaromatic amines. J Chem Soc Perkin Trans I:805–809 58. Katritzky AR, Yannakopoulou K, Lue P et al (1989) The chemistry of N-substituted benzotriazoles. Part 14. Novel routes to secondary and tertiary amines and N,N-disubstituted hydroxylamines. J Chem Soc Perkin Trans I:225–233 59. Corey EJ, Helal CJ (1996) Catalytic enantioselective synthesis of the second generation histamine agonist cetirizine hydrochloride. Tetrahedron Lett 37:4837–4840 60. Janetka JW, Furness MS, Zhang X et al (2003) Enantioconvergent synthesis of ()-(2R,5 S)1-allyl-2,5-dimethylpiperazine, an intermediate to d-opioid receptor ligands (2003). J Org Chem 68:3976–3980 61. Tomori H, Shibutami K, Ogura K (1997) Practical synthesis of (S)-3-(p-nitrobenzyloxycarbonylamino)pyrrolidine and its related compounds from -aspartic acid. Heterocycles 44:213–225

Top Curr Chem (2011) 299: 141–185 DOI: 10.1007/128_2010_82 # Springer-Verlag Berlin Heidelberg 2010 Published online: 13 August 2010

Synthesis of Neoclerodane Diterpenes and Their Pharmacological Effects Kimberly M. Lovell, Katherine M. Prevatt-Smith, Anthony Lozama, and Thomas E. Prisinzano

Abstract Salvinorin A is a neoclerodane diterpene that has been shown to be an agonist at kappa opioid receptors. Its unique structure makes it an attractive target for synthetic organic chemists due to its seven chiral centers and diterpene scaffold. This molecule is also interesting to pharmacologists because it is a non-serotonergic hallucinogen, and the first opioid ligand discovered that lacks a basic nitrogen. There have been several total synthesis approaches to salvinorin A, and these will be detailed within this chapter. Additionally, research efforts have concentrated on structure modification of the salvinorin A scaffold through semi-synthetic methods. Most modifications have focused on the manipulation of the acetate at C-2 and the furan ring. However, chemistry has also been developed to generate analogs at the C-1 ketone, the C-4 methyl ester, and the C-17 lactone. The synthetic methodologies developed for the salvinorin A scaffold will be described, as well as specific analogs with interesting biological activities. Keywords Neoclerodane diterpene Salvinorin A k Opioid Contents 1 2 3

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144 Total Synthesis Efforts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147 Semisynthetic Modifications of Salvinorin A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155 3.1 C2 Modifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155 3.2 Furan Ring Modifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163

K.M. Lovell and K.M. Prevatt-Smith contributed equally to this work. K.M. Lovell, K.M. Prevatt-Smith, and T.E. Prisinzano (*) Department of Medicinal Chemistry, The University of Kansas, Lawrence, KS 66045-7582, USA e-mail: [email protected] A. Lozama Division of Medicinal and Natural Products Chemistry, College of Pharmacy, The University of Iowa, Iowa City, IA 52242-1112, USA

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3.3 C1 Modifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171 3.4 C4 Modifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175 3.5 C17 Modifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178 4 Bioactivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178 5 Future Directions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182

Abbreviations Ac acac AIBN Ar BINAP Bn Boc BOM Bu Bz cat CC CDMT CNS Cp CT d dba DBAD DBU DCC DDQ DEAD DIAD DIBALH DMAP DMF DMPU DMSO DOB DOP dppf EDCI eq.

Acetyl Acetylacetonate 2,20 -Azobisisobutyronitrile Aryl 2,20 -Bis(diphenylphosphino)-1,10 -binaphthalene Benzyl tert-Butoxycarbonyl Benzyloxymethyl Butyl Benzoyl Catalyst cis-cis 2-Chloro-4,6-dimethoxy-1,3,5-triazine Central nervous system Cyclopentadienyl cis-trans Day(s) Dibenzylideneacetone Di-tert-butylazodicarboxylate 1,8-Diazabicyclo[5.4.0]undec-7-ene N,N-Dicyclohexylcarbodiimide 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone Diethyl azodicarboxylate Diisopropyl azodicarboxylate Diisobutylaluminum hydride 4-(Dimethylamino)pyridine Dimethylformamide 1,3-Dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone Dimethylsulfoxide 4-Bromo-2,5-dimethoxyamphetamine d Opioid receptor 1,10 -Bis(diphenylphosphino)ferrocene N-(3-Dimethylaminopropyl)-N0 -ethylcarbodiimide Equivalent(s)

Synthesis of Neoclerodane Diterpenes and Their Pharmacological Effects

Et Fmoc h HOBt HPLC Im i-Pr KHMDS KOP L LAH LHMDS LSD m-CPBA Me MEM Me-WMK min MNBA mol MOM MOP Ms NaHMDS NBS n-bu NCS NMM NMO PCC PDC Ph PPTS Pr PTSA Pv py rt s SAR s-Bu TBAF TBS TBSOTf

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Ethyl 9-Fluorenylmethoxycarbonyl Hour(s) 1-Hydroxybenzotriazole High pressure liquid chromatography Imidazole Isopropyl Potassium hexamethyldisilazide, potassium bis(trimethylsilyl)amide k Opioid receptor Liter(s) Lithium aluminum hydride Lithium hexamethyldisilazide, lithium bis(trimethylsilyl)amide Lysergic acid diethylamide m-Chloroperoxybenzoic acid Methyl (2-Methoxyethoxy)methyl (R)-()-Wieland–Miescher ketone Minute(s) 2-Methyl-6-nitrobenzoic anhydride Mole(s) Methoxymethyl m Opioid receptor Methanesulfonyl (mesyl) Sodium hexamethyldisilazide N-Bromosuccinimide n-Butyl N-Chlorosuccinimide 4-Methylmorpholine 4-Methylmorpholine N-oxide Pyridinium chlorochromate Pyridinium dichromate Phenyl Pyridinium p-toluenesulfonate Propyl p-Toluene sulfonic acid Pivaloyl Pyridine Room temperature Second(s) Structure-activity relationship sec-Butyl Tetrabutylammonium fluoride tert-Butyldimethylsilyl tert-Butyldimethylsilyl trifluoromethaneulfonate

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t-Bu TC TES Tf TFA TFAA THF THP TIPS TLC TMS Tol Ts TT

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tert-Butyl trans-cis Triethylsilyl Trifluoromethanesulfonyl (triflyl) Trifluoroacetic acid Trifluoroacetic anhydride Tetrahydrofuran Tetrahydropyran-2-yl Triisopropylsilyl Thin layer chromatography Trimethylsilyl 4-Methylphenyl Tosyl, 4-toluenesulfonyl trans-trans

1 Introduction Salvinorin A (1), a neoclerodane diterpene, is the active component of Salvia divinorum Epling & Jativa (Lamiaceae), a widely available psychoactive plant (Fig. 1) [1]. S. divinorum is a member of the sage family and is indigenous to Oaxaca, Mexico [1, 2]. It has been used for centuries by the Mazatec Indians in divination ceremonies as well as for headaches, rheumatism, and panzoń de barrego, a semi-magical disease [2]. Diterpene 1 was isolated from S. divinorum and determined to be a potent hallucinogen with an active dose in humans of 200– 1,000 mg when smoked, rivaling the potency of other classical hallucinogens such as lysergic acid diethylamide (LSD) and 4-bromo-2,5-dimethoxyamphetamine (DOB) [1, 3–6]. However, unlike LSD and DOB, 1 does not have activity at the serotonin 5HT2A receptor [5, 7]. Instead, 1 was found to be a selective k opioid (KOP) receptor agonist [4]. Salvinorin A (1) is the first non-nitrogenous natural product having high affinity and efficacy at KOP receptors [4]. Its unique structure has little structural similarity to other known opioid receptor ligands such as morphine (2), cyclazocine (3), fentanyl (4), SNC 80 (5), U50,488 (6), 3FLB (7), dynorphin A (8) (Fig. 2) [8–11]. All of these known opioid receptor ligands have a basic amino group, which was thought to be required for affinity. It was accepted that the positively charged nitrogen of the ligand interacts with an aspartate residue in the third transmembrane region of the opioid receptor, which is a G-protein coupled receptor [9, 12, 13]. Diterpene 1 lacks a basic amino group, which consequently led to reevaluation of the necessity of a basic nitrogen for opioid receptor affinity and efficacy [14]. In fact, recent work has found that other opioid ligands retain some affinity and efficacy at opioid receptors, even when the basic nitrogen is removed.

Synthesis of Neoclerodane Diterpenes and Their Pharmacological Effects O

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Fig. 1 Salvinorin A (1), clerodane skeleton (9), columbin (10), and clerodin (11)

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Tyr-Gly-Gly-Phe-Leu-Arg-Arg-Ile-Arg-Pro-Lys-Leu-Lys-Trp-Asp-Asn-Gln 8

Fig. 2 Morphine (2), cyclazocine (3), fentanyl (4), SNC 80 (5), U50,488 (6), 3FLB (7), and dynorphin A (8)

Neoclerodane diterpenes are members of the terpene class of natural products. Terpenes provide interesting chemical targets for organic chemists and pharmacologists. Their structural complexity and diversity, along with their unique pharmacological profiles, result in attractive structural scaffolds for investigation. Terpenes are secondary metabolites and are thus found in very low concentrations in living organisms [15]. They are biosynthesized from units of isoprene and are classified based on the varying number of isoprene units present in their carbon skeleton [15].

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Isoprene has the chemical formula of C5H8, therefore, each isoprene unit correlates to five carbons present in the skeleton. Terpene classification includes hemiterpenes, monoterpenes, sesquiterpenes, diterpenes, sesterterpenes, and triterpenes, each consisting of one through six isoprene units, respectively [16]. Additional terpene classifications include tetraterpenes, containing eight isoprene units, and polyterpenes, which include isoprene chains. The classification system is further divided into subclasses [15]. Specifically focusing on diterpenes, there are over 50 different subclasses of skeletons and many of these subclasses include compounds with biological activity [16]. Some of the most recognized diterpene subclasses include labdanes, kauranes, gibberlins, beyeranes, aphidicolins, cembranes, and abietanes. The clerodanes are another subclass of diterpenes that are known to have biological activity. Clerodanes (9) are ubiquitous throughout nature and are found in a variety of plants, fungi, and microorganisms (Fig. 1) [17]. A secondary metabolite is deemed a clerodane if it contains four isoprene units and four contiguous stereocenters on a cis or trans decalin ring system [17]. Approximately 25% of clerodanes contain the cis ring fusion and an example of this is columbin. Columbin (10), a diterpenoid furanolactone, has been isolated from several plants including Sphenocentrum jollyanum Pierre (Menispermaceae) and Jateorhiza columba Miers (Menispermacea) [18, 19]. It is sold in a crude drug preparation called Calumbae Radix or Tinosporae Radix [18]. Columbin itself has been shown to have dose dependent anti-inflammatory activity and chemopreventative activity against colorectal cancer [18–20]. The other 75% of clerodanes contain the trans ring fusion resembling clerodin. Clerodin (11) was originally isolated from Clerodendrum infortunatum L. (Lamiaceae) and is an insect antifeedant with potential as a natural pesticide [21–23]. Along with the relative configuration of the trans or cis junctions of the fused rings, clerodanes are further classified by their relative configuration at C-8 and C-9 [17]. This extra clarification results in four types of clerodane skeletons as defined with respect to configuration of ring fusion and substitution pattern at C-8 and C-9: transcis (TC), trans-trans (TT), cis-cis (CC) and cis-trans (CT) [17]. Clerodanes are further classified by their absolute stereochemistry. Compounds that have the same absolute stereochemistry as clerodin are termed neoclerodanes and compounds that are enantiomers of clerodin are referred to as ent-neoclerodanes [24, 25]. The biological activity and structural complexity of neoclerodane diterpenes have driven their study through total synthesis efforts and pharmacological investigation through biological and structure-activity relationship (SAR) studies. However, difficulties in conducting these studies result from the structural complexity, sensitivity to conditions of many typical organic reactions, potential problems in acquiring the natural product or plant material for extraction, and the unknown sites of action of neoclerodane diterpenes. The lack of precedent for chemical methodology required to transform neoclerodanes contributes to the limited SAR studies to date. Furthermore, developing reactions that are chemoselective for molecules that are often polyfunctionalized and contain multiple chiral centers is an extremely difficult undertaking. Determination of the molecular basis of activity through semi-synthetic means requires access to the natural product in substantial quantities,


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which can be difficult or impossible to obtain. Additionally, the site of action of neoclerodane diterpenes is often unknown, increasing the difficulty in SAR development. Despite these challenges, several neoclerodane diterpenes, including salvinorin A (1), have been investigated through total synthesis, semi-synthetic, and pharmacological studies. Salvinorin A is a tricyclic trans-cis neoclerodane diterpene containing seven chiral centers. Additionally, 1 contains multiple functional groups, which include four carbonyls at C-1, C-17, C-18, and C-21. Two of the carbonyls are esters off of the C-2 and C-4 positions of the decalin core, and the C-17 carbonyl is part of a d-lactone ring system. Additionally, 1 contains a furan ring off C-12. Along with the functionality present on 1, C-8 has been shown to undergo epimerization readily in acidic or basic conditions. Despite the molecular complexity, total syntheses of 1 have been accomplished and structural modifications have been completed. This chapter will address total synthetic attempts towards the molecule itself and synthetic methodologies developed and applied to modify the salvinorin A scaffold.

2 Total Synthesis Efforts The first published attempt at the total synthesis of 1 was by Lingham and coworkers [26]. They envisioned 1 being derived from highly functionalized ketone 12 and lactone 13 (Fig. 3). They believed that these two pieces could converge to form 1 through a Michael-type addition. The synthesis of 13 began with the reaction of 3-furylaldehyde with acetone and NaOH to give the racemic alcohol 14 in 97% yield (Scheme 1). Compound 14 was reacted with (Z)-2-bromobut-2-enoyl chloride to produce 15 in 92% yield. Treatment of this compound with Rieke zinc and acid afforded 13 as a 1:1 mixture of isomers in 92% yield. There are no additional reports towards the synthesis of 1 by this group to date. In 2007, the first successful total synthesis of 1 was reported by Evans and coworkers in 33 steps with 4.5% overall yield [27]. Evans envisioned 1 being derived from macrolactone 16 through a transannular Michael cascade (Fig. 4). Compound 16 would be derived from the fusion of vinyl iodine 17 and the highly functionalized aldehyde 18. The synthesis began when 3-furylaldehyde was reacted with propyne and oxidized with MnO2 to afford ketone 19 in 70% yield (Scheme 2). Reduction of 19 with (R)-b-methyl-oxazaborolidene gave the a-alcohol 20 in 85% yield. Using KH and H2N(CH2)3NH2, alkyne 20 was isomerized and followed with carboalumination and protection of the alcohol to give 17 in 68% yield. Compound 22 was produced in 85% yield from a Claisen condensation between 21 and ethyl hydrogen malonate. Lithium hexamethyldisilazide (LHMDS) and ClPO(OEt)2 followed by Fe-catalyzed cross-coupling produced 23 in 92% yield [28]. Compound 23 was involved in an aldol addition, alcohol protection, and acetylide addition to afford propargylic alcohol 24 in 83% yield. Compound 24 went through a series of reactions including protection, semi-hydrogenation, dihydroxylation, and ultimately oxidative cleavage to give 18 in 92% yield. A Grignard of 17 was

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Fig. 3 Retrosynthetic analysis of salvinorin A proposed by Lingham and co-workers

O

O AcO

H

H

O O

CO2Me 1

O O AcO O O

CO2Me 12

13

O O

H

OMe

O

O O O

OH a

b

H O

O O

O

c

Br

O

O

O

O 3-furylaldehyde

14

15

13

Scheme 1 (a) Acetone, NaOH; (b) (Z)-2-bromobut-2-enoyl chloride, Et3N, Et2O; (c) Rieke zinc in THF/H3O+

then reacted with 18 to give alcohol in 25 75% yield. Alcohol 25 was then silated with tert-butyldimethylsilyl trifluoromethaneulfonate (TBSOTf), and subsequently deprotected and hydrolyzed with LiOH to give 26 in 93% yield. Shiina


149

O

O AcO

H

H

O O

CO2Me 1

O O BOMO O

O O

CH(OMe)2 16

O

I

OTES 17

BOMO

CHO

OTBS CO2Me

CH(OMe)2 18

Fig. 4 Retrosynthetic analysis of salvinorin A proposed by Evans and co-workers

macrolactonization followed by removal of a silyl group and oxidation gave 16 in 95% yield [29, 30]. Treating 16 with tetrabutylammonium fluoride (TBAF) at 78 C, followed by warming to 5 C triggered the transannular cascade to give the tricyclic compound 27 as a single diastereomer in 95% yield. The enol of compound 27 was deoxygenated utilizing triflate formation followed by catalytic and conjugate reduction to give 28 in 78% yield. Compound 28 was subjected to deprotection, oxidation, and esterification to give 29 in 95% yield. K2CO3 in MeOH was used to epimerize C-8 and subsequent acylation afforded 1 in 78% yield. Many neoclerodanes, including 1 contain a trans-decalin core. In their attempt to synthesize 1, Burns and Forsyth have developed a synthetic strategy to establish the core through an intramolecular Diels–Alder/Tsuji allylation sequential sequence

150

K.M. Lovell et al. OH

O

CHO a

b

O

c

O 19

S

O N

20

OH

O

d, e

f, g

CO2Et

S

CO2Et CH(OMe)2

CH(OMe)2

22

23

24

O TBSO m

BOMO TBSO

H OH CO2H

l

TBSO

H

k

25 O

H O

O

o O

18

OTES CO2Me

CH(OMe)2

O

BOMO

H

BOMO

26

O

j

O OH

CH(OMe)2

n

OTBS CO2Me

h, i

CH(OMe)2 21

16

17

O

BOMO

H

H

O

H O

p, q O

O HO

H

H

H O

r, s

1

O

OH CH(OMe)2 27

CH(OMe)2 28

CO2Me 29

Scheme 2 (a): (1) Propyne, n-BuLi, Et2O, 78 C, (2) MnO2; (b) (R)-B-Methyl-CBS catalyst, BH3lMe2S, 30 C; (c): (1) KH, H2N(CH2)3NH2, 0 C; (2) Me3Al, Cp-2ZrCl2, I2, 45 C; (3) TESCl, imidazole; (d) Ni-(R)-BINAP(OTf)2, 2,6-lutidine, BF3lOEt2, HC(OMe)3; (e) HO2CCH2CO2Et, i-PrMgCl, 65 C; (f) LHMDS, ClPO(OEt)2; (g) Fe(acac)3, MeMgCl, 20 C; (h): (1) DIBALH, 78 C; (2) MnO2; (3) Sn(OTf)2, N-ethylpiperidine, (S)-3-acetyl-4-isopropylthiazolidine-2-thione, 78 C; (i): (1) TBSOTf, 2,6-lutidine; (2) K2CO3, MeOH; (3) OsO4, NMO, NaIO4; (4) Zn(OTf)2, ()-N-methyl-ephedrine, Et3N, 4-phenyl-1-butyne; (j): (1) BOMCl, NaHMDS, 78 C; (2) Lindlar catalyst, H2; (3) K2OsO4, NMO, citric acid, 50 C, Pb(OAc)4, K2CO3; (k) 17, n-BuLi, MgBrlOEt2, CH2Cl2, 78 to 0 C; (l): (1) TBSOTf, 2,6-lutidine; (2) PPTS, MeOH; (3) LiOH, i-PrOH, H2O; (m): (1) MNBA, DMAP, [0.0015 M]; (2) TBAF; (3) Dess–Martin periodinane; (n) TBAF, 78 to 5 C; (o): (1) NaH, Comins’ reagent; (2) Pd(OAc)2, dppf, Et3SiH; (3) L-selectride, t-BuOH, 78 to 55 C; (p) LiBF4, CH3CN/H2O; (q) NaClO2, TMSCHN2; (r) K2CO3, MeOH; (s) Ac2O, pyridine, DMAP

(Fig. 5) [31]. They believed that 1 could be derived from 30 through a series of protection, deprotections, and functionalizations. Intermediate 30 could result from highly functionalized ketone 31 through cyclization and Tsuji allyation. Reaction of the highly functionalized alkene 32 with Hg(ClO4)2, CaCO3 in a mixture of tetrahydrofuran (THF) and H2O at 0 C afforded 31 in 33% yield (Scheme 3). Heating 31 with toluene and butylated hydroxybenzene initiated an intramolecular Diels–Alder reaction to afford cyclic compounds 33 and 34 in 77% yield. Intramolecular Tsuji allylation was carried out using Pd2(dba)3 and PPh3 in toluene to give 30 in 86% yield and 35 in 61% yield, respectively. To date, Burns and Forsyth have


151

O O O AcO

H

H

H

O

O

O

O

O

O

O O

O O CO2Me 1

O 30

31

Fig. 5 Retrosynthetic analysis of salvinorin A by Burns and Forsyth

O O

O

O O

O

O a

31

b

H

O

O O

+

O

O

O

H

O

O

S S O

O 32

O O

O

33

34

c

c

H

30

O O

O

O

H

O

35

Scheme 3 (a) Hg(ClO4)2, CaCO3, THF/H2O; (b) toluene, butylated hydroxyl benzene; (c) Pd2(dba)3, PPh3, toluene

not published any further progress towards 1; however, their strategy does provide a way to achieve both cis and trans decalin cores which may have utility towards the synthesis of 1 or the synthesis of other neoclerodanes/natural products. In 2008, Hagiwara and co-workers reported the total synthesis of 1 [32]. They were able to achieve 1 in 20 steps with 0.95% yield employing a similar strategy they also used for the synthesis of ()-methyl barbascoate [33]. They envisioned that 1 could be achieved from enone 36 through a linear series of functionalizations beginning with the methyl analog of (R)-()-Wieland–Miescher ketone (Me-WMK) (Fig. 6). Starting with Me-WMK, protection, followed by reaction with NH4Cl in a mixture of KOH and MeOH gave 36 (Scheme 4). Compound 36 was then subjected to reductive alkylation and this afforded 37 and 38 in 25 and 51% yield, respectively. Compound 38 was carried forward and deprotected to afford

152

K.M. Lovell et al. O OH O AcO

H

H

O O

O O

O

O

O

CO2Me 1

36

Me-WMK

Fig. 6 Retrosynthetic analysis of salvinorin A proposed by Hagiwara and co-workers

CO2Et O a, b

O

c

36

O

O

HO

O

d, e

H

O

O

37

Me-WMK

O

+

O

O

CO2Et H

39

38

f-h O 12

MPMO H

H OH O

CHO O

MPMO H

OTBS O

o

OTBS

MPMO

MPMO

H

l-n

H

i-k

O

O O O

O

O

O 43 12R

42

41

40

+ O 12

MPMO H

O

H OH O

p-s

O

H

O 43 12S

H O

t, u O

O O

H

O

CO2Me 44

O HO

H

H

H O

v

1

O

CO2Me 45

Scheme 4 (a) (CH2)2(OTMS)2, Me3SiOTf, 15 Kbar, 40 C; (b) KOH, MeOH/H2O, O2, 65 C; (c) Li/NH3, THF, 78 C, alkyl iodide; (d) 3M HCl aqueous EtOH; (e) NaHMDS, Ph3PCH3Br, THF; (f) LAH, Et2O, 0 C; (g) TBSCl, DMAP, Et3N, CH2Cl2 (h) NaH, MPMCl, DMF; (i) BH3, THF, ˚ , CH2Cl2; (k) NaOMe, MeOH; (l) ethylene glycol, 3M NaOH, H2O2; (j) PDC, NaOAc, MS-4 A ˚ , CH2Cl2; (o) 3-bromofuran, t-BuLi, PTSA, 40 C; (m) TBAF, THF; (n) PDC, NaOAc, MS-4 A THF, 78 C; (p) PTSA, acetone, H2O, reflux; (q) DDQ, H2O, CH2Cl2, 0 C; (r) PDC, 2-methyl-2butene, DMF; (s) DCC, DMAP, MeOH, CH2Cl2; (t) NaHMDS, TESCl, THF; (u) m-CPBA, NaHCO3, toluene, H2O, 0 C, AcOH; (v) PPh3, DIAD, AcOH, CH2Cl2


153

the diketone and subsequently underwent double Wittig methylenation with sodium hexamethyldisilazide (NaHMDS) and Ph3PCH3Br to yield 39. Upon isolation of 39, it was immediately treated with lithium aluminum hydride (LAH) to afford the diol, which was itself protected, to produce 40 in 54% yield. Compound 40 was converted to the di-aldehyde 41 through hydroboration followed by oxidation in 94% yield. After protection of the formyl groups, deprotection of the silyl ether, and subsequent oxidation, 42 was converted to 43 in 78% yield. A 2:3 mixture of both the 12R and 12S isomer of 43 was achieved in 66% yield from the reaction of 42 with 3-lithiofuran. The 12S isomer of 43 underwent deprotection, oxidation and esterification to afford 2-desacetoxysalvinorin A (44) in 90% yield. Compound 44 was treated with NaHMDS and chlorotriethylsilane (TESCl) in THF at 78 C to give the silyl enol ether, which itself underwent Rubottom oxidation to yield 45 in 70% yield. Mitsunobu conditions were then employed to invert the C-2 stereochemistry and acetylate the hydroxyl to afford 1 in 86% yield [34, 35]. Recently, Hagiwara and co-workers have developed a second-generation total synthesis of 1 [36]. The first few steps are similar to the previous synthesis of 1, but they have been able to optimize their approach using a different strategy to complete the synthesis. Compound 38, the intermediate used in the previous generation synthesis, went through protection and deprotection steps followed by conversion of the carbonyls to enol triflates with Comins’ reagent and NaHMDS in THF to afford 46 in 66% yield (Scheme 5). The ester group of 46 was converted to the Weinreb amide and reacted with 3-furyllithium to give 47 in 70% yield. Treatment of 47 with Pd(PPh3)4, dppf, carbon monoxide and triethylamine (Et3N) at 60 C in a 3:1 mixture of MeOH and dimethylformamide (DMF) afforded the bisester 48 in 84% yield. Treating 48 with K-selectride resulted in reduction and concomitant ring closure to give 49 in 95% yield with the 12(S) configuration as the sole diastereomer. The two alkenes present in 49 were reduced using samarium iodide in the presence of Et3N, acetic acid, and toluene followed by an O2 quench to produce 50 in 64% yield. Deprotection, C-8 epimerization, and Dess–Martin oxidation produced 44, which was transformed to 1 in similar fashion as the first generation synthesis. Overall, the second generation synthesis of 1 by Hagiwara reduced the number of steps from 20 to 13 steps and increased the overall yield from 0.95 to 2.8%. In the attempt to synthesized 20-norsalvinorin A (51) Perlmutter and co-workers prepared a tricyclic intermediate (52) [37]. They chose to synthesize this analog to determine the importance of the methyl group at C-20. They envisioned that 51 could be achieved from modification of this advanced intermediate (Fig. 7). Compound 52 was thought to be derived from 53. The synthesis begins with complexion of 54 with TiCl4 at 78 C, which was followed by the addition of 53 (derived from Snapper’s protocol in three steps from 3-furylaldehyde) [38] to afford 55 in 37% yield (Scheme 6). Compound 55 was treated with 1,8-diazabicyclo-[5.4.0]undec-7-ene (DBU) at room temperature in CH2Cl2 to give 56 in 87% yield. This transformation was carried out in order to induce the correct stereochemistry at C-10 of the tricyclic core. The alkene at the 2,3 position of 56 was reduced selectively using Na2S2O4 and

154

K.M. Lovell et al. O

O HO

O

CO2Et H

CO2Et

TESO H

O

OTf

a-c

O

TESO

e, d

O OTf

H

f

TESO

O CO2Me

H

OTf

38

OTf 47

46

CO2Me 48 g

O

1

O

l, m

H

H

O

H O

i-k

TESO H

H

O

H O

O

h

H O

TESO H

O

CO2Me 44

O

CO2Me

CO2Me

50

49

Scheme 5 (a) 3M HCl aqueous EtOH; (b) TESOTf, pyridine, DMAP, DMF, 100 C; (c) Comins’ reagent, NaHMDS, THF; (d): (1) i-PrMgBr, THF; (2) MeN+(OMe)H2Cl; (e) 3-bromofuran, t-BuLi, THF; (f) Pd(PPh3)4, CO, dppf, MeOH/DMF; (g) K-selectride, t-BuOH, THF; (h) (1) SmI2, Et3N, AcOH; (2) toluene, O2 quench; (i) TBAF, THF; (j) K2CO3, MeOH (k) Dess–Martin, CH2Cl2; (l) (1) NaHMDS, TESCl, THF; (2) m-CPBA, NaHCO3, toluene, H2O, 0 C, AcOH; (m) PPh3, DIAD, AcOH, CH2Cl2

O

O O

O AcO

H

H H

CO2Me 51

O

O O

MeO

O

H

O H

O

O 52

53

Fig. 7 Retrosynthetic analysis of 20-nor-salvinorin A proposed by Perlmutter and co-workers

NaHCO3 in a mixture of toluene and H2O, without the loss of the methoxy group at C-2 to afford the desired intermediate 52 in 73% yield. While having successfully synthesized their desired intermediate, to date Bergman and co-workers have not reported any further progress towards the synthesis of 51 or 1.


155

O

O

O MeO

O

a MeO

O

b MeO

O

O

H

H

O

54

O

H

MeO

O

H

H O

55

O

c

H O

56

52

Scheme 6 (a): (1) TiCl4, toluene, 78 C; (2) 53, 45 C; (b) 10% DBU, CH2Cl2; (c) Na2S2O4, NaHCO3, toluene/H2O, Adogen 464

3 Semisynthetic Modifications of Salvinorin A 3.1

C2 Modifications

In addition to the total syntheses of salvinorin A (1), semi-synthetic structural modifications to 1 have been performed to establish SAR. The most extensively studied position on the salvinorin A core is the C-2 ester followed closely by the furan ring. Further investigations have been performed at C-1, C-4, and C-17 as well. The quantity and diversity of analogs synthesized at C-2 is partly due to the ability to hydrolyze selectively the C-2 ester to the C-2 hydroxyl intermediate, salvinorin B (57). While retaining the stereochemistry of the stereocenters of 1, selective removal of the C-2 acetate has been achieved by treating 1 with Na2CO3 in MeOH to give salvinorin B (57) in 77% yield (Scheme 7) [39, 40]. Several other attempts have been made to achieve this intermediate such as ammonolysis at 0 C in MeOH. These conditions also resulted in the synthesis of 57 but this method resulted in low yields and significant epimerization of C-8 [39]. The epimerization of C-8 is thought to occur readily during deacetylation of C-2 due to breaking the C-8/C-9 bond via base-promoted cleavage [41, 42]. A number of diverse organic and inorganic bases have been tried but have resulted in no overall improvement in the synthesis of 57 [41]. KOH in MeOH under O2 resulted in the oxidation of 1 and gave products 58 and 59 in 53% and 37% yield, respectively [43]. However, refluxing 1 in 5% aqueous KOH resulted in 60 in 95% yield [44, 45]. When Ba(OH)2 was used as the base in MeOH, the oxidative-elimination product 61 was formed in 75% yield [41]. Also attempted was refluxing KCN in MeOH/THF to achieve the deacetylated product; this resulted in a 51% yield of the 8-epi-57 [46]. The C-2 hydroxyl derivative with the unnatural configuration (2-Epi-57 or 62) was synthesized using Mitsunobu conditions of triphenylphosphine (PPh3), diisopropyl azodicarboxylate (DIAD), and 4-nitrobenzoic acid as the nucleophile, which was subsequently hydrolyzed with K2CO3 in MeOH to afford 62 in 64% yield over two steps [34]. Compounds 57 and 2-epi-57 have been exploited as common intermediates in diversification of the C-2 position. Hydroxyl 57 has been utilized

156


O

R

H

O

O

O R1O

O

H

H

O O

R2O CO2Me

O

58 R = OH 61 R = H

CO2Me

R1 = Me; R2 = H R1 = H; R2 = Me 59

c, d, or f O

O HO

H

H

O

O

O O

g, h

AcO

H

H

O

O

O O

CO2Me

CO2Me

62

a or b

HO

H

H

O O

CO2Me 57

1 e

O

O HO

H

OH

O

H

OH O

OH 60

Scheme 7 (a) Na2CO3, MeOH; (b) NH3, MeOH, 0 C; (c) KOH, MeOH, O2; (d) Me3SiCHN2 (e) 5% aqueous KOH, 80 C; (f) Ba(OH)2, MeOH; (g) 4-nitrobenzoic acid, PPh3, DIAD, CH2Cl2; (h) K2CO3, MeOH

in the synthesis of esters, carbamates and carbonates, ethers and amines, amides and thioesters, and sulfonyl esters and sulfonamides. Preparation of C-2 esters was performed by subjecting 57 to a number of standard acylating procedures with slight deviation in the base used (Scheme 8) [34, 41]. Common procedures include the use of the appropriate acid chloride or anhydride with a catalytic amount of 4-(dimethylamino)pyridine (DMAP) in CH2Cl2 to yield a number of aryl and alkyl esters (63) [34, 39, 41, 47]. Furthermore, 57 has been exposed to conditions coupling in appropriate carboxylic acids with


157

O

O R

O

H

O

H

O

O CO2Me 65 R = Me 66 R = Ph

d or e O

O HO

H

H

O

O

O

a or b O

R

O

H

H

O O

O CO2Me

CO2Me

57

63 R = alkyl, aryl

c

O

O H

O

H

H

O O

O CO2Me 64

Scheme 8 (a) Appropriate alkyl chloride or anhydride, DMAP, CH2Cl2; (b) appropriate acid, EDCI, HOBt, CH2Cl2; (c) formic acid, acetic anhydride; (d) AcOH, PPh3, DIAD, CH2Cl2; (e) appropriate acid, (2-Py)PPh2, DBAD, THF, 60 C

N-(3-dimethylaminopropyl)-N0 -ethylcarbodiimide (EDCI) and 1-hydroxybenzotriazole (HOBt) in CH2Cl2 (63) [48]. Ester derivatives were also prepared from 2-epi-57 by alkylating with appropriate acid chloride or anhydrides and Et3N in CH2Cl2 [34]. In addition, 57 was treated with a mixture of formic acid and acetic anhydride (Ac2O) to yield the C-2 formate (64) [49].

158

K.M. Lovell et al.

The C-2 epimer of 1 was synthesized from 57 using Mitsunobu conditions of PPh3, DIAD, and acetic acid in a 38% yield (65) [34]. However, the C-2 acetyl and benzyl C-2 epimers of 1 also have been prepared using modified Mitsunobu conditions [50]. Diphenyl-2-pyridylphosphine and either acetic acid or benzoic acid were dissolved in THF followed by the addition of di-tert-butylazodicarboxylate (DBAD) at 60 C to yield analogs 65 and 66 in 80% and 75% yield, respectively [50]. Alkyl carbonates were synthesized by treating 57 with methyl or ethyl chloroformate and a catalytic amount of DMAP in CH2Cl2 to afford 67 and 68, respectively (Scheme 9) [41]. Compound 69 was generated from 57 and trichloroacetyl isocyanate in CH2Cl2 followed by in situ hydrolysis with aluminum oxide in 91% yield over two steps [51]. Additional carbamates were synthesized by treating 57 with methyl isocyanate or ethyl isocyanate, pyridine, and a catalytic amount of DMAP in CH2Cl2 in 68% (70) and 53% (71) yield, respectively [51]. Compounds 72 and 73 were obtained by treating 57 with phenylisocyanate or allylisocyanate in CH2Cl2 resulting in a 14% yield and 17% yield, respectively [39].

O

R

O

H N

O

H

H

O O

O CO2Me 69 R = H 70 R = Me 71 R = Et

72 R = Ph 73 R = allyl

b, c or d or e O

O HO

H

H

O

O

O

a O

R

O

O

H

H

O O

O CO2Me

CO2Me

57

67 R = Me 68 R = Et

Scheme 9 (a) Chloroformate, DMAP, CH2Cl2; (b) Cl3C(O)NCO, CH2Cl2; (c) Al2O3; (d) appropriate isocyanate, DMAP, pyridine; (e) appropriate isocyanate, TMSCl, CH2Cl2


159

Appropriate alkyl iodides or bromides were reacted with 57 and 2-epi-57 in the presence of Ag2O in CH3CN with yields ranging from 36 to 77% (74, Scheme 10) [34, 51]. However, with these conditions preparation of branched alkyl ethers was unsuccessful [51]. Reacting 57 with chloromethyl methyl ether, diisopropylethylamine, and catalytic DMAP in CH2Cl2 afforded 75 in a 72% yield and was the first 1 derivative reported to have increased affinity at the KOP receptor relative to 1. [41, 52]. Based on the successful increase in affinity of derivative 75, additional simple alkoxymethyl ethers were obtained using appropriate alkyoxymethyl chloride with diisopropylethylamine in DMF (76–78). However, more complex alkoxymethyl derivatives were synthesized from the common methylthiomethyl ether intermediate (79), which was obtained from reaction of 57 with acetic acid, acetic anhydride, and dimethylsulfoxide (DMSO) [52]. Compound 79 was then O

O

O F

H

O

H

O

O

e

O

S

H

H

O

O

O

d O

O

O

R

O

81

H

O O

CO2Me

CO2Me

H

CO2Me

79

80 R = alkyl

c

O

O R

H

H

O

O

f

O HO

O

H

a

O

O R

O

CO2Me

O

H

H

O O

CO2Me

CO2Me

57

74 R = unbranched alkyl chain

R O ∗ O

H

O

82 b

O

O

83

O

O ∗ O

O

84 R

O

H

H

O O

CO2Me 75 R =

O

77 R =

O

76 R =

O

78 R =

O

TMS Ph

Scheme 10 (a) RX, Ag2O, CH3CN; (b) ROCH2Cl, i-Pr2NEt, DMF; (c) Ac2O, AcOH, Me2SO; ˚ sieves; (f) PPTS or PTSA, CH2Cl2 ˚ sieves; (e) NIS, Et2NSF3, 4-A (d) NIS, ROH, TfOH, 4-A

160

K.M. Lovell et al.

exposed to alcoholysis in the presence of N-iodosuccinimide (NIS) and catalytic trifluoromethanesulfonic acid (TfOH) to afford the desired ethers (80) [52]. These conditions were ineffective for the synthesis of the fluoromethyl ether analog 81, and conditions were modified to include NIS and Et2NSF3 [52]. Additional alkoxyalkyl ethers were synthesized using ethoxyethene (82), 2-methoxypropene (83), and 3,4-dihydro-2H-pyran (84) with catalytic pyridinium p-toluenesulfonate (PPTS) in CH2Cl2 [52]. Substituted amines with the unnatural configuration at C-2 were obtained by activation of the C-2 alcohol of 57 with trifluoromethanesulfonic anhydride and nucleophilic displacement with desired amine (85, Scheme 11) [34, 51]. Amines that retained the C-2 configuration were synthesized using the same methodology starting with 2-epi-57 to yield 86 [34]. The synthesis of the primary amine at C-2 began with 57 which was treated with the desired thionyl halide to yield the C-2 halogen (87, Scheme 12) [34, 53, 54]. The C-2 halogen was then displaced by O

O HO

H

H

O

O

a, b R2

O

R2 1 O H R N

H

O O

CO2Me

CO2Me

57 2-epi-57

85 R1 = α-H 86R1 = β-H R2 = alkyl or hydrogen

Scheme 11 (a) (CF3SO2)2O, pyridine, CH2Cl2; (b) R2NH, THF O

O

O HO

H

H

O

O O

a or b

X

CO2Me

H

O

H

O O

c or d

N3

H

O

57R = β-H 2-epi-57R = α-H

b, d or g, h

O N3

H

91

O

H

e or f

H2N

H

H

O O

CO2Me

89

88 O

O

O O

CO2Me

O

H

CO2Me

O

O

CO2Me

H

O

O

X = Br, Cl 87 O

R

O

H

CO2Me

57

HO

O

i, j

H2N

H

H

O O

CO2Me

90

Scheme 12 (a) SOX2, Et3N, ClCH2CH2Cl; (b) CBr4, PPh3, CH2Cl2; (c) NaN3, DMSO; (d) NaN3, DMF, AcOH; (e) TMSCl, NaI, CH3CN; (f) Zn, NH4Cl, MeOH, CH2Cl2; (g) (CF3SO2)2O, pyridine, CH2Cl2; (h) NaNH3, DMF; (i) PPh3; (j) H2O, THF


161

nucleophilic attack of sodium azide to yield azide 88 [34, 53]. Azide 88 was then reduced with TMSCl and sodium iodide in CH3CN to give the primary amine (89) in 5% yield over three steps [34]. Improvement of the procedure to obtain the primary amine was achieved by using CBr4, PPh3, and CH2Cl2 to afford the brominated product 87 in 59% yield [48]. Yield of the nucleophilic displacement of bromide with sodium azide was increased to 86% by modifying the solvent to a mixture of DMF and acetic acid (88) [48]. Reduction of azide 88 was then carried out with Zn metal and NH4Cl and yielded amine 89 in 36% yield, an 18% yield over three steps [48]. Additionally, 2-epi-57 was submitted to identical conditions, leading to the generation of azide 91 [48]. Finally, 57 has been activated with trifluoromethanesulfonic anhydride and displaced with nucleophilic attack of sodium azide to afford 91, which was reduced by Staudinger conditions to afford amine 90 in a 35% yield over three steps [34]. Secondary and tertiary amides were synthesized by subjecting previously prepared primary and secondary amines (86 and 89) to appropriate acid chloride or anhydride with Et3N and optional DMAP in CH2Cl2 (92 and 93) (Scheme 13) [34, 48]. C-2-Epi-amides were also synthesized using C-2 epimerized amines 85 and 90 under the same conditions (94 and 95) [34]. To synthesize thioester derivatives, the C-2 halogen (87) was treated with the potassium salt of thiobenzoic acid or thioacetic acid in CH3CN or acetone to yield

O

O

R2 R2

N

R1

O

H

R2

a

O

H

N

O

R1

O

H

O

H

O

O CO2Me

CO2Me

86 R1 = β-H 85 R1 = α-H R2 = alkyl or hydrogen

92 R1 = β-H 94 R1 = α-H R2 = alkyl or hydrogen

O

H2N

R1

O

H

H

O

O O

a

H R1 N

R2

O

H

H

O O

O CO2Me 89 β-H 90 R1 = α-H R1 =

CO2Me 93 R1 = β-H 95 R1 = α-H R2 = alkyl or acyl

Scheme 13 (a) Appropriate acid chloride or anhydride, DMAP, NEt3, CH2Cl2

162

K.M. Lovell et al.

thioesters 96 and 97, respectively (Scheme 14) [48, 53]. In addition, the reaction of the C-2 chloride with sodium hydrogen sulfide (NaSH) in MeOH and DMF afforded compound 98 [54]. However, low yield and formation of side products resulted in amendments to proceed through thioacetate 96 and then deacetylate with sodium thiomethoxide in MeOH and CH2Cl2 to afford 98 [54]. Finally, 2-epi-96 can be achieved by triflating 57 and subsequent treatment with potassium thioacetate in acetone at 20 C [54]. Sulfonyl esters were synthesized by treating 57 with appropriate sulfonyl chlorides with a catalytic amount of DMAP in CH2Cl2 to afford the desired sulfonyl

O

O X

H

H

O

O

O

a or b R

O

H

S

H

O O

O

96 R = Me 97 R = Ph

CO2Me

CO2Me

87 X = Cl, Br e

d O

O X

H

H

O

O

O

c HS

O

H

H

O

CO2Me

CO2Me

87 X = Cl, Br

98

O

O HO

H

O

H

O

O

O

f, g S

O

H

H

O O

O CO2Me

57

CO2Me

2-epi-96

Scheme 14 (a) RCOSK, CH3CN; (b) potassium thioacetate, acetone; (c) NaSH, MeOH, DMF; (d) NaSMe, MeOH, CH2Cl2, 20 C; (e) Ac2O, DMAP, CH2Cl2; (f) (CF3SO2)2O, pyridine, CH2Cl2, 20 C; (g) potassium thioacetate, acetone, 20 C


O

O X

H

H

163

O

a O

O R O

CO2Me 57 or 89 X = OH, NH2

S

X

H

H

O O

O CO2Me 99 X = O; R = Me 100 X = O; R = Ph 101 X = O; R = 4-MePh 102 X = NH; R = Me 103 X = NH; R = Ph

Scheme 15 (a) Appropriate sulfonyl chloride, DMAP, CH2Cl2

esters (99–101) in 32–86% yield (Scheme 15) [39, 47]. Along with sulfonyl esters, sulfonamides were synthesized by utilizing previously prepared amine 89 and subjecting it to methanesulfonyl chloride or benzenesulfonyl chloride with a catalytic amount of DMAP in CH2Cl2 to provide sulfonamides 102 and 103 in 56% and 97% yields, respectively [48]. Another modification made off the C-2 position of 1 is the trichloroacetimidate derivate (104), which was prepared by treating 57 with trichloroacetonitrile and DBU in ClCH2CH2Cl at 0 C to yield product 104 in a 37% yield (Scheme 16) [39]. Efforts were also made to remove the C-2 hydroxyl of 57 entirely. Hydroxyl 57 was treated with 1,1-thiocarbonyldiimidazole and catalytic DMAP in CH2Cl2 to afford imidazole 105 in 64% yield [50]. Imidazole 105 was then refluxed with 2,20 -azobis (2-methylpropionitrile) (AIBN) and tributyltin hydride in toluene to bestow C-2 dehydroxylated 57 in a C-8 epimeric mixture of C-8 a-H and b-H in 13 and 27% yield (106 and 107), respectively [50]. Additionally, dilactone 108 was obtained by treating 57 with a mixture of chromium trioxide and pyridine in CH2Cl2 in a 37% yield [50].

3.2

Furan Ring Modifications

It has been shown that natural products containing a furan ring are potentially hepatotoxic [55]. This toxicity is thought to arise from oxidation of the furan ring to an enedial by cytochrome P450 enzymes. The enedial can subsequently react with biological nucleophiles and form stable conjugates. Thus, there has been a relatively large amount of research interest concentrated on elucidating replacements for the furan ring in the salvinorin A scaffold that will not only allow for exploration of SAR, but also result in the production of compounds with reduced potential for hepatotoxicity.

164


O N

O

N

H

H

O

O c

O

H

R

O

O

O

S CO2Me

CO2Me 106 R1 = α-H 107R1= β-H

105 b

O

O HO

H

H

O

O

O

a O

Cl3C

O

H

H

O O

NH CO2Me

CO2Me 57

104

d

O

O

H

O

H

O O

CO2Me 108

Scheme 16 (a) Trichloroacetonitrile, DBU, ClCH2CH2Cl, 0 C; (b) (Im)2CS, DMAP, CH2Cl2; (c) AIBN, Bu3SnH, toluene; (d) CrO3, pyridine

Catalytic hydrogenation of salvinorin A (1) in MeOH over 5% Pd/C quantitatively afforded a stereoisomeric mixture (at C-13) of products resulting from hydrogenolysis of the lactone (109, Scheme 17) [1]. Carboxylic acid 109 was esterified with diazomethane to yield methyl ester 110 [42]. While treatment of 1 with sodium borohydride has been found to lead to extensive epimerization at C-8, it was found that methyl ester 110 was relatively stable towards epimerization at


O

H

O

O

O

H

165

O

a

H

O

O

O

H

O

OH

O

b O

O

O

H

H

O O

O

CO2Me

CO2Me

1

CO2Me 110

109

c

O

O O

H

O

O

H

O

+

O

O

O

H

H

OH O

O CO2Me 111

CO2Me

Scheme 17 (a) H2, Pd/C, MeOH; (b) CH2N2/MeOH, 0 C; (c) H2, Rh/C, CH2Cl2/MeOH

O

O

HO O

O OH

O O

H

H

O

O O

O

O

H

1

O

O

+ O

O

O CO2Me

H

O H

H

O O

O CO2Me 112

CO2Me 113

Scheme 18 Photoxidation in the presence of light

C-8, allowing the use of sodium borohydride for the reduction of the C-1 ketone. The use of rhodium catalyst in place of palladium was found to minimize the hydrogenolysis of the lactone and allowed access to tetrahydrosalvinorin A (111) [49]. The C-13R epimer of 111 was separable by HPLC [49, 56]. Exposure of 1 to sunlight in CH2Cl2 and MeOH yielded the natural products salvidivin A (112) and salvidivin B (113) in 6.7 and 18.2% yield, respectively (Scheme 18) [56]. The treatment of 1 with bromine in a mixture of CH2Cl2 and MeOH at 30 C gave the 2,5-dimethoxydihydrofuran product (114) in 93% yield as a mix of cis and trans isomers, which were separable by HPLC (Scheme 19) [50, 56]. As a mixture,

166

K.M. Lovell et al. MeO O HO HO O

H

O

OMe O

H

O

O

MeO

MeO O

CO2Me 115

O O

O

H

O

H

H

O

O O

O

O

HO HO

OMe c

b

MeO

O

O OMe H

H

O O

O

OMe O

H

CO2Me

CO2Me

114

117

O d

O a CO2Me

O

O

116

S

N Br O O

O

e O O

H

O

O

H

R

O

O O

H

H

O

O O

H

O O

O

O

O

H

CO2Me 119 120 121

CO2Me

CO2Me 122

1

R = Me R = Ph R = 4-OMePh

f

O

O Br

O O

H

H

g

O

O O

O

O

H

H

O O

O CO2Me 124

CO2Me 123

Scheme 19 (a) Br2, MeOH, CH2Cl2, 30 C; (b) KMnO4, THF/H2O, 10 C; (c) H2, Rh/C, MeOH; (d) appropriate sulfonamide, HOAc, 95 C; (e) Br2, DMF; (f) NBS, CH3CN; (g) tributylvinyltin, Pd(PPh3)4, toluene, 80 C

the cis isomer of 114 was selectively oxidized over the trans isomer with KMnO4 in THF in H2O at 10 C to afford a mixture of the natural products salvinicin A (115) and salvinicin B (116) in an approximate 3:2 ratio, respectively. These were readily separable from each other and from the unreacted trans isomer by flash column chromatography. Dimethoxydihydrofuran 114 was also catalytically reduced with hydrogen and 5% Rh/C to give dimethoxytetrahydrofuran 117 in 92% yield as a mixture of compounds containing the 2,5-dimethoxy cis and trans isomers [57].


167

The purification of this mixture has been accomplished by HPLC [56]. As a mixture, the treatment of 117 with methanesulfonamide at 95 C in acetic acid afforded a mixture of the methanesulfonylpyrrole (119, 30% yield) and its C-8 epimer (25% yield) [57]. Using this methodology, alkyl- and arylsulfonamides (119–121) have been prepared in 23–30% yield, but with significant amounts of the C-8 epimers also formed (18–20% yield). The treatment of 1 with bromine in DMF at 0 C afforded a dihydrofuran product (122) as a mixture of C-15 a and b isomers instead of the expected 2-bromofuran product (Scheme 19) [50]. However, treatment of 1 with N-bromosuccinimide (NBS) in acetonitrile gave the desired 2-bromofuran product (123) in 38% yield. From 123, Stille coupling with tributylvinyl tin in the presence of Pd(PPh3)4 afforded the 2-vinylfuran product (124) in 58% yield [44]. The reaction of 1 with NaIO4 and a catalytic amount of RuCl3l3H2O in a mixture of CCl4, CH3CN, and H2O resulted in oxidative degradation of the furan ring, providing the carboxylic acid at C-13 (125) in 74% yield (Scheme 20) [50]. Carboxylic acid 125 was converted to the thioester in 88% yield by treatment with 2-chloro-4,6-dimethoxy-1,3,5-triazine (CDMT) and N-methylmorpholine, followed by ethanethiol [58]. The thioester derivative was reduced with triethylsilane and Pd/C to afford the aldehyde at C-13 (126) in 82% yield [58]. Generation of the carboxylic acid and aldehyde at the C-13 position of the salvinorin A scaffold has granted access to a variety of chemical methods for the development of salvinorin A analogs with a C-13 carbonyl moiety. From carboxylic acid 125, alcohols and ethers at C-13 have been synthesized [44]. The primary alcohol at C-13 (127) was produced in 46% yield by reducing carboxylic acid 125 using BH3lTHF (Scheme 21). Alcohol 127 was then alkylated using alkyl iodides in the presence of Ag2O to afford alkyl ethers (128 and 129) in 12–15% yield. The primary alcohol was also acetylated (130) in 11% yield with acetic anhydride and Et3N. Different conditions have been used to access a variety of esters at the C-13 position (Scheme 21). The methyl ester of carboxylic acid 125 was prepared in 48% yield by treatment of 125 with (trimethylsilyl)diazomethane (TMSCHN2) [44]. Further alkyl esters (131) have been produced in 26–54% yield using appropriate alcohols in the presence of EDCI and DMAP, or in 40–72% yield using appropriate alcohols in the presence of EDCI, HOBt, and Et3N [44, 58]. A large variety of

O

O O

H

H

O O

O CO2Me 1

O

a

O

H

O

OH

O

H

O

O

b, c

O

O

O

H

H

H

O O

O CO2Me 125

CO2Me 126

Scheme 20 (a) RuCl3, NaIO4, CCl4/CH3CN/H2O; (b) CDMT, NMM, EtSH, CH2Cl2; (c) Pd/C, Et3SiH, CH2Cl2

168

K.M. Lovell et al. OH O O

H

H

O

OR O

b or c

O

O

O

H

H

O O

O CO2Me 127

128 129 130

CO2Me R = Me R = Et R = Ac

a

O O O

H

XR

H

O d, e , f, g, or h

O

O O

O

O

H

OH

H

O O

O CO2Me

CO2Me

125

131 X = O; R = alkyl, aryl, heterocycle 132 X = NH; R = alkyl, aryl, heterocycle

i, j

O

O

HO

O

H

R

O

S

O

133

134

135

R= H

O

O

k O

H

H

O

N

O

N

O

O CO2Me 138

N

CO2Me 136

137

Scheme 21 (a) BH3lTHF, THF, 55 C; (b) R-I, Ag2O, CH3CN, 60 C; (c) Ac2O, Et3N, CH2Cl2; (d) TMSHN2, MeOH, toluene; (e) R-OH, EDCI, DMAP, CH2Cl2; (f) R-OH, EDCI, HOBt, Et3N, CH2Cl2; (g) R-NH2, EDCI, HOBt, CH2Cl2; (h) R-NH2, EDCI, HOBt, Et3N, CH2Cl2; (i) (COCl)2, CH2Cl2; ( j) Bu3SnR, Pd(PPh3)4, toluene, 80–100 C; (k) NaBH4, MeOH, 0 C

amides (132) have been produced from carboxylic acid 125 in 45–72% yield using appropriate amines in the presence of EDCI and HOBt [44], or in 18–75% yield using appropriate amines in the presence of EDCI and HOBt with the addition of Et3N (Scheme 21) [58]. Carboxylic acid 125 has also been useful for the generation of ketones at the C-13 position. Conversion of carboxylic acid 125 to the acyl chloride was achieved using oxalyl chloride (Scheme 21) [44]. The crude product was carried through and coupled to appropriate aryltributyl tin reagents under Stille conditions to afford a variety of arylketones (133–137) in 7–57% yield [44, 59]. The 2-furylketone compound (133) produced with this methodology was subsequently reduced using sodium borohydride to afford the corresponding alcohol (138) as a mixture of C-13 epimers in 19% yield. B-Allyl-(10S)-(trimethylsilyl)-9-borabicyclo[3.3.2]decane (prepared in situ from commercially available 9-[(1R,2R)-pseudoephedrinyl]-(10S)-(trimethylsilyl)-9borabicyclo[3.3.2]decane and allylmagnesium bromide) achieved the asymmetric


O

O

H

F3C O

H H

O

169

H

O a

O

O

O

H

H

OH H O

OMe H

b O

O

O

O

O CO2Me

H

H

O H O O

O

CO2Me

CO2Me 126

139

140

c, d O

H O O

H

H

O H O O

O CO2Me 141

Scheme 22 (a) 9-(1R,2R-Pseudoephedrinyl)-(10S)-(trimethylsilyl)-9-borabicyclo[3.3.2]decane, allylMgBr, Et2O; (b): (1) (R)-(+)-a-methoxy-a-trifluoromethylphenylacetic acid, oxalyl chloride, benzene; (2) DMAP, Et3N, CH2Cl2; (c) acryloyl chloride, DMAP, Et3N, CH2Cl2; (d) Grubbs II, CH2Cl2

allylation (139) of the C-13 aldehyde (126) in 82% yield (Scheme 22) [58]. In order to determine the stereochemistry of the allylic alcohol product, the Mosher ester (140) was prepared in 82% yield using oxalyl chloride and (R)-(+)-a-methoxy-atrifluoromethylphenylacetic acid. Allylic alcohol 139 was acylated with acryloyl chloride in the presence of Et3N and DMAP, and the subsequent ester was subjected to ring closing metathesis using second generation Grubb’s catalyst to afford the 5,6-dihydro-2H-pyran-2-one product (141) in 23% yield over two steps. Aldehyde 126 was converted to the terminal alkyne (142) in 61% yield by Seyferth–Gilbert homologation (Scheme 23) [60]. These basic conditions were well tolerated and no C-8 epimerization or C-2 deacetylation was observed. Alkyne 142 was coupled with various azides under copper(I)-catalyzed click reaction conditions to afford 1,4-disubstituted-1,2,3-triazoles (143–145) in 88% yield. Similar conditions were used to generate unsubstituted 1,2,3-triazoles. Alkyne 142 was coupled to azidomethyl pivalate under click reaction conditions to give the pivalyl triazole (146) in 88% yield. The pivalyl triazole was then treated with 1N NaOH to remove both the pivalyl group and the C-2 acetyl group. These conditions also led to C-8 epimerization. Acetylation of the epimeric mixture under standard conditions (Ac2O, pyridine) and preparative scale separation by TLC furnished the C-8 epimeric N-unsubstituted triazoles (147) in 54% yield over three steps in a ratio of 4:6 (C-8:epi-C-8).

170

K.M. Lovell et al. R O O O

H

N N N

H

H

O

O

a

O

O

O

H

H

O

b

O

O

O

H

H

O O

O O CO2Me

CO2Me

126

CO2Me 143 R = Me 144 R = Et 145 R = Bn

142 c

O N NH N O O

H

H

O N N N

O

d O

O O

O CO2Me 147

H

H

O O

O CO2Me 146

Scheme 23 (a) Ohira-Bestmann reagent; (b) RN3CuSO4 (cat.), sodium asorbate, t-BuOH/H2O; (c) azidomethyl pivalate, CuSO4 (cat.), sodium asorbate, t-BuOH/H2O; (d): (1) 1M NaOH (2.2 eq.), MeOH/H2O, (2) 1M HCl (2.2 eq.), (3) Ac2O, pyridine

Removal of the furan ring (148) was accomplished in two steps in 45% overall yield from carboxylic acid 125 (Scheme 24) [57]. Carboxylic acid 125 was coupled with selenophenol via the mixed anhydride using phenyl dichlorophosphate to give the phenyl seleno ester. Decarbonylation of the phenyl seleno ester was accomplished with tributyltin hydride and AIBN in toluene. Carboxylic acid 125 was coupled with ethanolamine or L-serine methyl ester hydrochloride in the presence of EDCI, HOBt, and Et3N to afford the corresponding b-hydroxy amides, which were cyclized using bis(2-methoxyethyl)aminosulfur trifluoride (Deoxo-Fluor) to afford oxazolines (149 and 150, Scheme 24) [56, 57]. Oxazoline 150 was converted to oxazole 151 in 80% yield by treatment with bromotrichloromethane and DBU, conditions which did not lead to epimerization at C-8 [57]. A variety of oxadiazoles (152–158) have been produced in 10–56% yield by treating carboxylic acid 125 with appropriate amidoximes in the presence of EDCI and HOBt followed by heating in toluene (Scheme 24) [44, 57, 58]. These conditions led to some epimerization at C-8, necessitating purification by HPLC. Carboxylic acid 125 was treated with either (2-hydroxybenzyl)triphenyl- or (2-thiobenzyl)triphenyl phosphonium bromide in the presence of CDMT and Et3N to afford the benzofuran (159) and benzothiophene (160) products in 26 and 35% yield, respectively (Scheme 24) [58]. In addition to the large variety of replacements for the furan ring, the stereochemistry of C-12 has been inverted. Heating 1 in 5% aqueous KOH afforded the diacid in 95% yield (161, Scheme 25) [44]. The free hydroxyl at C-2 was


171

X O O

H

H

O

N O

O O CO2Me 159 160

X=O X=S

H

H

H

O

CO2Me

MeO2C

OH

N

O O

a, b

O

O

O

H

H

O

c or d, e

O

O

O

O CO2Me

f

R

O

H

MeO2C

N O

N

O

O O

g, h

R = Me R = Et R = iPr R = Pr R = Ph R = 4-FPh R = Bn

H

CO2Me 150

125

152 153 154 155 156 157 158

H

O

O

CO2Me

148

O

149 or O

O

H

O

i

O

O

O

H

N

O

O O

O

O

H

O

H

O O

O CO2Me

CO2Me 151

Scheme 24 (a) PhOPOCl2, PhSeH, Et3N, THF; (b) Bu3SnH, AIBN, toluene; (c) EDCI, HOBt, ethanolamine, Et3N, CH2Cl2; (d) EDCI, HOBt, L-serine methyl ester hydrochloride, Et3N, CH2Cl2; (e) deoxo-fluor, CH2Cl2; (f) BrCCl3, DBU, CH2Cl2; (g) EDCI, R(NH2)=NOH, CH2Cl2; (h) toluene, heat; (i) CDMT, Et3N, appropriate Wittig reagent, CH2Cl2/toluene, reflux

re-acetylated (162) with Ac2O and pyridine in 48% yield. Refluxing in acetic acid promoted cleavage at C-1 and lactonization to afford the inverted center at C-12, and subsequent methylation of the acid at C-4 using TMSCHN2 provided 12-episalvinorin A (163) in 19% yield over two steps.

3.3

C1 Modifications

Synthetic methodologies have been explored and developed for selective access to the C-1 ketone for the production of analogs. When 1 was treated with

172


O O

H

H

O

O

O CO2Me 1

O

a O

HO

H

OH

O

H

OH

O

b O

O

H

H

OH O

OH

O

CO2H

CO2H

161

162

c, d

O

O O

H

H

O O

O CO2Me 163

Scheme 25 (a) 5% KOH (aq.), 80 C; (b) Ac2O, pyridine; (c) AcOH, 118 C; (d) TMSCHN2, CH3CN

sodium borohydride, the cis-1,2-diol (164) was produced in 40% yield (Scheme 26) [1, 61]. Unfortunately, these conditions also led to significant epimerization at C-8, also producing the stereoisomeric diol in 40% yield. Using standard conditions (Ac2O, pyridine), the C-2 hydroxyl was selectively acetylated to give the C-2 monoacetate (165). The C-1 hydroxyl was selectively acetylated in 83% yield with trimethyl orthoacetate in the presence of catalytic acetic acid at 100 C to afford the C-1 monoacetate (166) [61]. Subsequent acetylation of monoacetate 166 under standard conditions yielded the 1, 2-diacetate (167) in 94% yield. Diol 164 was treated with 1,10 -thiocarbonyldiimidazole to produce the cyclic thiocarbonate (168) product in 67% yield over two steps from salvinorin A (Scheme 26) [49]. Unfortunately, these conditions were found to cause epimerization at C-8, the products of which were inseparable on silica gel. However, this meant that initial separation of the C-8 epimers of the cis-diol starting material was not necessary. Radical reduction (tributyltin hydride, AIBN, toluene, 80 C) of the epimeric thiocarbonate mixture afforded the separable C-1 deoxy products in 47% combined yield. Subsequent acetylation of the C-2 hydroxyl under standard conditions (Ac2O, pyridine) gave 1-deoxysalvinorin A (169) in 82% yield.


173

O

O

S O

H

O

e, b

O

H

H

O

O

H

O O

O CO2Me

CO2Me 168

169

d O

O O

H

H

O

a

O O

HO HO

H

H

O

b

O

HO AcO

O

H

H

O O

O CO2Me

CO2Me

CO2Me

164

165

1

c

O

AcO HO

H

H

O

b

O O

AcO AcO

H

H

O O

CO2Me

CO2Me

166

167

Scheme 26 (a) NaBH4, i-PrOH; (b) Ac2O, pyridine; (c) trimethyl orthoacetate, AcOH (cat.), 100 C; (d) thiocarbonyldiimidazole, DMF, 90 C; (e) Bu3SnH, AIBN, toluene, 80 C

Recently, the conditions for the hydride reduction of the C-1 ketone were improved (Scheme 27). Treating 1 or herkinorin (170, for synthetic conditions see Scheme 8) with an aqueous solution of sodium borohydride in THF selectively reduced the C-1 ketone (171 and 172) in 77 or 45% yield respectively, with no evidence of C-8 epimerization [62]. Treatment of 1a-hydroxysalvinorin A (171)

174


O

O RO

H

H

a

O

HO RO

O

H

H

O

b

O

O O

O

O S

O

H

H

O

c

O O

O HO

O S

O

H

H

O O

O CO2Me 1 R = Ac 170 R = COPh

CO2Me

CO2Me

CO2Me

171 R = Ac 172 R = COPh

173

174

b, h

d

O

H

RO

O

O

or O

H

O

CO2Me 181 R = Ac 182 R = COPh

H

O

e or g

O

H

HO

O

CO2Me

O O

CO2Me

177

175 +

O

R

O

O

g

O O

H

HO

O

f

O O

H

O

O O

O CO2Me

CO2Me

CO2Me

178

176

179 R = Me 180 R = Ph

Scheme 27 (a) NaBH4, THF/H2O; (b) (CH3SO2)2O, DMAP, CH3CN; (c) NaOH, MeOH, CH2Cl2, 10 C; (d) DMAP, DMSO, 170 C; (e) MnO2, toluene; (f) NaBH4, CH3CN; (g) Ac2O or benzoyl chloride, DMAP, CH2Cl2; (h) trimethylphenylammonium chloride, CH3CN

with methansulfonic anhydride and DMAP afforded 1a-mesyloxysalvinorin A (173) in 99% crude yield. Mesylate 173 was hydrolyzed at C-2 by NaOH in MeOH/CH2Cl2 at 10 C to give crystalline 1a-mesyloxysalvinorin B (174) in 70% overall yield from 1a-hydroxysalvinorin A (171). 1a-Mesyloxysalvinorin B (174) was then treated with DMAP in DMSO at 170 C to give a mixture of 1-deoxo-1,10-dehydrosalvinorin B (175, 76% yield) and 2-keto-1-deoxosalvinorin A (176, 22% yield). Allylic alcohol 175 was oxidized with MnO2 in toluene to afford the a,b-unsaturated ketone (177) in 78% yield. The reduction of 2-keto-1deoxosalvinorin A (176) with sodium borohydride gave 1-deoxosalvinorin B (178) in 41% yield. Acylation of 1-deoxosalvinorin B (178) was accomplished through treatment with acetic anhydride or benzoyl chloride in the presence of DMAP to afford 1-deoxosalvinorin A (179) and 1-deoxoherkinorin (180), respectively. Treatment of 1a-hydroxysalvinorin A (171) with methanesulfonic anhydride followed


175

by heating with trimethylphenylammonium chloride furnished the dehydrated product (181) in 76% yield. Finally, 1-deoxo-1,10-dehydrosalvinorin B (175) was treated with benzoyl chloride in the presence of DMAP to give 1-deoxo-1, 10-dehydroherkinorin (182).

3.4

C4 Modifications

Modifications of 1 have also been performed to modify the methyl ester off C-4 (Scheme 28). Diterpene 1 was treated with lithium ethanethiolate in 1,3-dimethyl3,4,5,6-tetrahydro-2(1H)-pyrimidinone (DMPU); however, in addition to nucleophilic cleavage of the C-4 methyl ester, these conditions cleaved the C-2 acetate. Therefore, standard acetylating conditions were employed to yield acids 183 and 184 due to the mixture of C-8 epimers [49]. Conditions were then modified to selectively cleave the C-4 acid by treating 1 with lithium iodide in pyridine (183 and 184) resulting in a 1:1 mixture of the C-8 epimer [63]. The C-8 epimeric mixture of acids 183 and 184 were subjected to standard esterification conditions such as N,N0 -dicyclohexylcarbodiimide (DCC), catalytic DMAP in CH2Cl2 with desired alcohols including alkyl, alkenyl, and terminal alkynes (185 and 186) [63, 64]. After esterification, the C-8 epimers were separated by chromatography. Additionally, the C-18 methoxyethoxymethyl (187 and 188) and methoxymethyl (189 and 190) derivatives were synthesized with chloromethoxyethoxymethyl ether (MEMCl), iPr2NEt in CH2Cl2 or MOMCl and Et3N in CH2Cl2, respectively [63, 64]. C-4 Amides were prepared by using EDCI, DMAP, and DMF or EDCI, HOBt, Et3N, and DMF (191 and 192) [34, 63]. Amides synthesized include the addition of alkyl, alkenyl, benzyl, amino acid, and terminal hydroxyl residues to the C-18 position [63]. Specifically, the C-4 acid with natural C-8 configuration was treated with 3-aminopropanenitrile to yield amide 193 [64]. Amide 193 was then treated with PPh3, azidotrimethylsilane (TMSN3), and DIAD in THF to afford the rearranged product and C-4 salvinorin A acid isostere 194 in a 29% yield over two steps [64]. The C-4 acids (183 and 184) have also been subjected to borane reduction conditions to afford alcohol 195 in 23–50% yield or 64% yield as the C-8 epimeric mixture (195 and 196, Scheme 29) [34, 49, 64]. The C-8 alcohol epimers 195 and 196 have been treated separately as a common intermediate for a number of C-4 derivatives including esters, ethers, and amines [34, 49, 64]. Alcohols 195 and 196 was subjected to DCC, DMAP, and desired acid chloride or carboxylic acid in CH2Cl2 affording ester analogs in 50–92% yield [64]. Esters prepared include alkyl, aryl, and fluorenylmethyloxycarbonyl (Fmoc) protected amino acid derivatives (197 and 198) [64]. Ethers were prepared with various alkyl halides and Ag2O in CH3CN at 40 C. Alkyl, allyl, and benzyl ethers were prepared in 45–80% yield (199 and 200) [34, 64]. Alcohols 195 and 196 were then activated to the triflates and displaced by a variety of amines by treatment with trifluoromethanesulfonic anhydride and desired amine in 22% – quantitative yield over two steps (201 and 202)

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K.M. Lovell et al. R1

R2

185 β-H

alkyl, alkenyl, terminal alkynes

O

O AcO

R1

H

186 α-H

O O

187 β-H

OR2

O

O

O

188 α-H 189 β-H

O

190 α-H d or e or f

O

O AcO

H

H

O

O

O O

O

a, b or c AcO

OMe

H

R

O

O

O O

O

g or h AcO

OH

N R2

R2

191 R1 = β-H 192 R1 = α-H R2 = alkyl, alkenyl, benzyl

i

O

O AcO

H

H

O

O

O O

O

N H 193

O O

O

183 R = β-H 184 R = α-H

1

R1

H

CN

j

AcO

H

H

O O

N N N N

CN

194

Scheme 28 (a) LiSEt, DMPU, 55 C; (b) Ac2O, DMAP, pyridine; (c) LiI, pyridine, 120 C; (d) ROH, DCC, DMAP, CH2Cl2; (e) MEMCl, i-Pr2NEt, CH2Cl2; (f) MOMCl, Et3N, CH2Cl2; (g) EDCI, HOBt, Et3N, DMF; (h) EDCI, DMAP, DMF; (i) 3-aminopropanenitrile, EDCI, HOBt, DMF; (j) PPh3, TMSN3, DIAD, THF


177

O

O AcO

H

O

R

O

O

O

O

OH

AcO

183 R = β-H 184 R = α-H a g O

O AcO

R1

H

R

O O

O 203 R = β-H 204 R = α-H

O

O

O O

N R2

H

e, f

AcO

R1

H

O

O

O O

R2

b or c

AcO

O O

O

O

H

R1

R2

197 R1= β-H 198 R1 = α-H R2 = aryl, alkyl, alkene, Fmoc-protected amino acids

d

AcO

O O

OH 195 R1= β-H 196 R1 = α-H

201 R1= β-H 202 R1 = α-H R2 = H or alkyl

R1

H

O O

OR2 199 R1= β-H 200 R1= α-H R2 = alkyl, allyl, benzyl

Scheme 29 (a) BH3-THF, 55 C; (b) appropriate acid, DCC, DMAP, CH2Cl2; (c) appropriate acyl chloride, DMAP, CH2Cl2; (d) RX, Ag2O, MeCN, rt or 40 C; (e) (CF3SO2)2O, pyridine, CH2Cl2, 0 C; (f) appropriate amine, THF; (g) Et3N, DMSO, (COCl)2, 60 C

178


O O

H

H

O

O

O

a O

O

H

O

O

H

O

O

O

c or d O

OH

H

O

CO2Me

CO2Me

1

O

O

+

O

H

O

O CO2Me

205

H

O

207

CO2Me 208

b

O

O O

H

O OMe

O CO2Me 206

Scheme 30 (a) DIBALH, THF, 78 C; (b) PPTS, MeOH; (c) Et3SiH, BF3lEt2O, CH2Cl2, 0 C; (d) Et3SiH, Amberlyst 15 resin, CH2Cl2

[34]. Additionally, alcohols 195 and 196 were oxidized to the aldehydes in 75% yield (203 and 204) [64].

3.5

C17 Modifications

Few analogs have been generated at the C-17 position. Salvinorin A (1) was converted to the C-17 lactol (205) in 65% yield by treatment with diisobutylaluminum hydride (DIBALH) (Scheme 30) [49]. Treatment of the lactol 205 with PPTS in MeOH afforded the methoxy derivatives (206) as an epimeric mixture that was separable by silica gel column chromatography [65]. 17-Deoxysalvinorin A (207) was synthesized in 48% yield by deoxygenation of lactol 205 using Et3SiH and BF3lEt2O [49]. The C-8 – C-17 enol ether (208) was formed in 23% yield as a byproduct of these reaction conditions. The substitution of Amberlyst 15 resin for BF3lEt2O in the reaction conditions provided the enol ether (208) as the sole product in 76% yield.

4 Bioactivity Salvinorin A is an agonist at KOP receptors [4]. Medicinal chemistry efforts have explored the SAR of salvinorin A, and have generally focused on improving the affinity and selectivity for KOP receptors. The SAR known at this time is


179

summarized in Fig. 8. At the C-1 position, reduction (165) or removal (169) of the carbonyl is tolerated (Fig. 9, Table 1). Interestingly, replacement of the acetyl group in 169 with a benzoyl group (180) resulted in an antagonist at m opioid receptors (MOP), KOP receptors, and d opioid receptors (DOP) (Fig. 9, Table 2). Introduction of an alkene at C-1 – C-10, exemplified by 181 (Fig. 9, Table 2), is also likely to impart antagonist activity. At the C-2 position, bioisosteric replacements of the ester functionality, such as amides, carbonates, and carbamates, are tolerated. In general, small alkyl groups favor affinity at KOP receptors. The methoxymethyl (75) and ethoxymethyl (76) ethers at this position are among the most potent salvinorin A derived KOP agonists reported to date (Fig. 9, Table 1). Aromatic groups at C-2 tend to favor affinity at MOP receptors. Examples of this change in

Removal reduces affinity. Reduction is tolerated. Degradation or replacement is tolerated.

O

Reduction or removal is tolerated. C12 inversion lowers efficacy. 12 1, 10-alkene is likley to be an antagonist. O O H H O Bioisosteric replacements are tolerated. 1 17 O Reduction or removal is tolerated. 2 Small alkyl groups favor KOPs. 8,–17 alkene is tolerated. O 4 Aromatic groups favor MOPs. CO2Me Reduction or hydrolysis reduces affinity. Small alkyl esters are preferred.

Fig. 8 Summary of the structure-activity relationships of salvinorin A analogs O

O

O

O (R)

H O O

H

H

O

O O

O

O

H

H

O

O O

O

O

H

H

O

O O

O

O

H

H

H O O

O CO2Me

CO2Me

CO2Me

CO2Me

1

75

76

111-13R

O

O O

H

H

O

O

HO O

O

O

H

H

O

O

O

H

O

O

H

O

O

O

O CO2Me

CO2Me

163

165

169

O

CO2Me

O

H N

O

O

H

H

O

H

O

O

O

H N

O

O

O

170

O

O

H

O

CO2Me

O

H

O

O

O

H

H

O

O O

H

O

O O

O

CO2Me

CO2Me

CO2Me

CO2Me

180

181

209

210

Fig. 9 Structures of interesting biologically active salvinorin A derivatives

H

O

O

Table 1 Opioid receptor affinity, potency, and efficacy of salvinorin A analogs Compound MOP Ki SD (nM) DOP Ki SD (nM) KOP Ki SD (nM) 1 >1,000 >5,970 980 1.9 0.2 75 ND ND 0.4 0.02 76 ND ND 0.32 0.02 111 13R 9,790 1,090 >10,000 3.7 0.2 163 ND ND 41 5 165 ND ND 1,125 365 169 ND ND 18 2 170 12 1 1,170 60 90 2 209 3.1 0.4 810 30 7,430 880 210 >10,000 3,090 220 >10,000 EC50 SEM (nM) (k) 40 10 (k) 0.6 0.2 (k) 0.14 0.01 (k) 750 60 (k) 84 8 ND (k) 141 43 (m) 500 140 (m) 360 60 ND

Emax SEM (%) (k) 120 2 (k) 98 ND (k) 81 7 (k) 67 5 ND (k) 122 27 (m) 130 4 (m) 134 5 ND

Refs. [39] [41] [52] [56] [44] [49] [49] [47, 48] [48] [58]

180 K.M. Lovell et al.

Synthesis of Neoclerodane Diterpenes and Their Pharmacological Effects Table 2 Inhibition of agonist stimulated [35S]GTP-g-S binding Compound – Ke SD (nM) – m, DAMGO d, DPDPE 180 4,700 910 8,780 2,130 181 200 30 400 90

– k, U69,593 580 30 570 140

181

Refs. – [62] [62]

affinity are herkinorin (170), the first non-nitrogenous selective MOP receptor agonist described, and the related benzamido herkinorin (209, Fig. 9, Table 1). At the C-4 position, reduction or hydrolysis of the ester reduces affinity. Replacement of the methyl ester with small alkyl groups is tolerated, whereas replacement with larger groups reduces affinity. At the C-17 position, reduction or removal of the lactone is tolerated. Subsequent methylation of the lactol after reduction of the lactone has little effect on affinity at KOP receptors. Additionally, introduction of a C-8 – C-17 alkene was tolerated. For the furan ring, removal reduces affinity. However, reduction to the tetrahydrofuran (111–13R, Fig. 9, Table 1) is tolerated. Inversion of the stereochemistry at the C-12 position (163, Fig. 9, Table 1) produces a partial agonist at KOP receptors. Finally, degradation of the furan ring and/or replacement with other heterocycles is tolerated. In fact, exploration at this point in the molecule has produced the first DOP selective ligand derived from the salvinorin A scaffold (210, Fig. 9, Table 1).

5 Future Directions Future research involving salvinorin A will uncover more high affinity, selective ligands for opioid receptors. Even with the wealth of currently available SAR information, there are still some significant gaps in the literature. Collective research has produced a large variety of salvinorin A-based selective KOP receptor agonists. However, there are comparatively few selective KOP receptor antagonists. Further, although herkinorin (170, Fig. 9) has been identified as a high affinity, selective MOP receptor agonist, there are no salvinorin A-based selective MOP receptor antagonists, nor are there any high affinity ligands selective for the DOP receptor. Identification of these currently missing salvinorin A-based opioid receptor ligands would not only provide interesting biological probes, but would also potentially lead to the discovery of new opioids for therapeutic use in the clinic. Currently, the C-2 position and the furan ring (C-13) of salvinorin A are still the most manipulated sites on the scaffold. At the C-2 position, much research has focused on bioisosteric replacements of the ester moiety. However, given the identification of the high affinity and potency methoxymethyl (75) and ethoxymethyl (76) ether analogs (Scheme 10), the C-2 position merits an investigation of conformational constraint. At the C-13 position, the only synthetic methodology developed to date for the formation of direct carbon-carbon bonds is Stille coupling conditions (Scheme 21). Future methodologies directed at this position would

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benefit from improving upon these conditions by eliminating the need for tin reagents. Finally, very little research has been conducted to scrutinize the core ring structure of the salvinorin A scaffold. Total synthesis efforts have provided several ways to arrive at the neoclerodane core ring structure, but there have been no SAR investigations to probe, for example, the necessity of the B-ring, or of the methyl group off C-9. Further explorations of the SAR of salvinorin A would benefit from the systematic introduction of flexibility into this relatively highly constrained scaffold, and new synthetic methodologies are needed in order to achieve this. Over the past decade, we have witnessed unparalleled advances in our understanding of the basic biological processes that contribute to many human diseases. However, lack of detailed understanding of the underlying etiologies of many central nervous system (CNS) disorders offers a unique opportunity for chemists working on opioids in collaboration with pharmacologists. Expanding the exploration of molecules with opioid receptor activity is likely to identify novel active agents that may serve as drug leads and new chemical tools to understand better the etiology of complex CNS disorders. Acknowledgments The authors thank the National Institute on Drug Abuse, the National Institutes of Health, and the University of Kansas for financial support of ongoing research programs. The content is the sole responsibility of the authors and does not necessarily represent the official view of the National Institute on Drug Abuse or the National Institutes of Health. K.M.L. and K.M.S. thank the NIH Dynamic Aspects of Chemical Biology Training Grant for pre-doctoral fellowships.

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Synthesis of Novel Basic Skeletons Derived from Naltrexone Hiroshi Nagase and Hideaki Fujii

Abstract We will describe eight interesting reactions using naltrexone derivatives. Almost all these reactions are characteristic of naltrexone derivatives, and can lead to the synthesis of many novel skeletons that provide new interesting pharmacological data. Some of the new reactions that were found with naltrexone derivatives were expanded into general reactions. For example, the reaction of 6a-hydroxyaldehyde derived from naltrexone led to the oxazoline dimer and the 1,3,5-trioxazatriquinane skeleton (triplet drug); this reaction was applied to general ketones which were converted to a-hydroxyaldehydes, followed by conversion to dimers and trimers, as described in Sect. 7. Keywords Intramolecular reaction Naltrexone Oligomerization Opioid Rearrangement reaction

Contents 1 2 3 4 5 6 7 8

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188 Synthesis of a Novel Ortho Ester [16] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190 Synthesis of Opioid Ligands with Oxabicyclo[2.2.2]octane and Oxabicyclo[2.2.1] heptane Skeletons [23] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195 Novel Cleavage Reaction of the C16–N17 Bond in Naltrexone Derivatives [26] . . . . . . . 200 A Double Decarboxylation Reaction of an Oxazolidinone and a Carboxylic Acid: Its Application to the Synthesis of a New Opioid Lead Compound [45] . . . . . . . . . . . . . . . . 203 Synthesis of a Stable Iminium Salt and Propellane Derivatives [43] . . . . . . . . . . . . . . . . . . . . 210 Novel Synthesis of a 1,3,5-Trioxazatriquinan Skeleton Using a Nitrogen Clamp [87] . . . . 215 New Rearrangement Reaction for the Synthesis of a New Opioid Ligand with an Oxabicyclo[3.2.1]octane Skeleton [92] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223

H. Nagase (*) and H. Fujii School of Pharmacy, Kitasato University, 5-9-1, Shirokane, Minato-ku, Tokyo, Japan e-mail: [email protected]

188


9

Novel Rearrangement Reaction for Converting a 6,14-Endoethanomorphinan Derivative to a Benzomorphan Derivative [97] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 227 10 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234

Abbreviations ACE COSY CPM GPCR HMBC HSQC MVD NBO NOE NTI TCE Troc

1-Chloroethoxycarbonyl Correlation spectroscopy Cyclopropylmethyl G-protein-coupled receptor Heteronuclear multiple bond coherence Heteronuclear single quantum correlation Mouse vas deferens Natural bond orbital Nuclear Overhauser effect Naltrindole 1,1,2,2-Tetrachloroethane 2,2,2-Trichloroethoxycarbonyl

1 Introduction Since morphine was first isolated by the German pharmacist Sertu¨rner [1] and its structure was independently determined by Gulland and Robinson [2, 3] and Scho¨pf [4], many research groups have studied its skeleton, its conversion, and its total synthesis. Morphine has a unique 4,5-epoxymorphinan skeleton that comprises some sequential asymmetric centers and a phenolic hydroxy group, a 4,5-epoxy ring, and a basic nitrogen. This structure has given rise to many complex and novel reactions. Many of the early reactions were described in detail in two books published in 1986, entitled Opioid Analgesics; Chemistry and Receptors [5] and Opiates [6]. After that, only a few studies focused on 14-hydroxy-4,5-epoxymorphinan represented by naloxone and naltrexone (1), and those were mostly limited to intermolecular reactions, although Portoghese et al. described many interesting reactions [7–12] with naltrexone (1). Naltrexone (1) has a 14-OH group in addition to the 4,5-epoxymorphinan structure, which comprises four sequential asymmetric centers with two OH groups, and a basic nitrogen. This structure has led to many complex intramolecular reactions. In fact, in 1989 and 1990, we showed that the 14-OH group participated in several characteristic intramolecular reactions [13, 14].

Synthesis of Novel Basic Skeletons Derived from Naltrexone H N17

O 14

H 7

H N

O 14

O O

1

189

H 7

H N

OH

OO

OH

2

O O

OH

OAc

OH

N

N

Ac2O py, rt

O OAc

4

OH

3

O

NaOH MeOH

O OH

OAc

OH

5

Scheme 1 The 14-OH group of naltrexone (1) participates in a reaction that facilitates the enolization of 6-ketone OH

O N

N O OMs 6

OMs

t-BuOK DMSO

O 7

OH

Scheme 2 Intramolecular SN2 reaction between the 14-OH group and the 6a-mesylate of naltrexone derivative 6

Naltrexone (1) is less polar than 14-H naltrexone due to the hydrogen bond between the 14-hydroxy group and the 17-nitrogen (Scheme 1). The basic nitrogen readily extracts the 14-OH proton to give an alkoxide ion 2, which accelerates the enolization of 6-ketone to give enolate 3. The enolate 3 reacts with acetic anhydride-pyridine at room temperature to afford triacetate 4. Furthermore, when 4 is treated with a base, an intramolecular acetyl migration reaction occurs that produces b-diketone (enol form) 5 [14]. Another example of an intramolecular SN2 reaction with the 14-OH group is shown in Scheme 2 [15]. Thus, the reactivity of naltrexone (1) is different from that of 14-H morphinans. More recently, we described several other intramolecular reactions of naltrexone (1). The focus of this chapter is on our recent studies on naltrexone (1), including intramolecular reactions with the 14-OH group and oligomerization reactions. Figure 1 shows several structures derived from naltrexone (1) with various types of reactions, including bond scission, cyclization, and rearrangements. Some reactions occur due to intramolecular interactions with the 14-OH group. First, we will describe the reactions that involve 14-OH participation, then we will describe other reactions. We will also describe some oligomerization reactions that produce two

190

H. Nagase and H. Fujii O

O

N

OH

R N

O

N

O

HO

Me

R N O O

O O

OH

OH

OH

R N

N N

O O

O O

R

O

X

O O OH

O OH

N OH

Me N H

OH

O O

O OH

OH

OH Naltrexone (1)

OH

N

OH

OH

OH

Fig. 1 Novel basic structural skeletons of naltrexone (1) derivatives

types of dimers and one trimer (Fig. 2). These products are converted to various derivatives that have interesting pharmacological effects on opioid receptors. Unfortunately, due to the limitation of space, we cannot describe all the reactions and their pharmacological effects. Therefore, we will discuss eight characteristic reactions of naltrexone (1).

2 Synthesis of a Novel Ortho Ester [16] Buprenorphine and etorphine are potent m and e opioid analgesics. Like opioid analgesics in general, they are capable of causing serious respiratory depression. Moreover, the side effects produced by these two particular analgesics are particularly dangerous because they cannot be reversed with the m opioid antagonist, naloxone [17]. This resistance to naloxone antagonism is generally attributed to their high affinity for the m receptor and their high lipophilicity [18]. TAN-821 is a potent, selective e opioid receptor agonist that produces strong analgesia in vivo [19]. However, it is not highly selective for the e receptor; in fact, it has been found to bind to the m opioid receptor in vitro (mouse vas deferens (MVD) assay). These three compounds all have a skeleton that comprises 6,14-endoethanotetrahydrothebaine with a 7,8-ethylene bridge. We hypothesized that their strong affinity for the

Synthesis of Novel Basic Skeletons Derived from Naltrexone

191

OH N

CHO O OH OMe

HO

MeO

OH O

N

O

O

HO O N

N

N O

O

OO(S) H OMe

O

N OH

OH N

OH HO H O O OH H

OO

N

N O O

OH

H OH

HO N

Fig. 2 Synthesis of two dimers and a trimer from naltrexone 6-hydroxy-6-aldehyde

m opioid receptor may derive, in part, from the high lipophilicity conferred by this bridge. If true, it should be possible to reduce their affinity for the m opioid receptor by introducing a hydrophilic group. To test this hypothesis, we designed a novelcompound 8a with a hydrophilic bridge. This compound 8a was called 17-(cyclopropylmethyl)-4,5-epoxy-3,6-dihydroxy-8-oxa-6,14-endoethnomorphinan (Fig. 3). We expected compounds derived from 8a to show improved e selectivity compared to TAN-821 in vitro. Initially, we planned to synthesize the hemiacetal 8b or acetal 8c with an intramolecular cyclization between the 14-OH and 6-aldehyde groups in a-hydroxyaldehyde 9. We reasoned that 9 could be obtained by reacting naltrexone methyl ether with a carbanion of 1,3-dithiane, followed by oxidative hydrolysis of thioacetal 10 (Fig. 4). However, when we attempted to obtain 9 from the hydrolysis of 10 with CuCl2/CuO or HgCl2/CdCO3 in aqueous MeCN or CH3COCH3 [20, 21], we

192

H. Nagase and H. Fujii Me

Me

OH

N

OH

Me N OMe

OMe

O OH Buprenorphine

O OH etorphine

O N Me

N

OR2

O N

OH

OH

O

O OR1 8a (R1 = R2 = H) 8b (R1 = Me, R2 = H) 8c (R1 = Me, R2 = alkyl) 8d (R1 = Me, R2 = Me)

OH TAN-821

Fig. 3 Structures of buprenorphine, etorphine, TAN-821, and derivatives of compound 8

OH N

OH CHO OH O

N

O OH

OMe 9

S S

OMe 10

Fig. 4 The structures of a-hydroxyaldehyde 9 and thioacetal 10

were unsuccessful. We next tried subjecting 10 to an acetal exchange reaction with CuCl2/CuO (2 and 4 equiv.) and CH(OMe)3 in MeOH at 50 C; from this we expected to obtain the dimethyl acetal 11. Instead, surprisingly, we obtained a substantial yield (55%) of the ortho ester 12. This was the first time that an oxidation method succeeded in introducing a 6b-carboxylic acid functional group into naltrexone (1) without the oxidation of any other functional groups (e.g., N or OH). Moreover, this approach produced the first 8-oxabicyclo[2.2.2]octane derivative 12 to be described in opioid chemistry. We therefore sought to find conditions that optimized this reaction (Fig. 5). We first ran the reaction with metals commonly used to catalyze the hydrolysis of thioacetal, including HgCl2/CdCO3, AgNO3/AgO, and FeCl3/CuO. We found that the hydrolysis (oxidation) proceeded with FeCl3/CuO and, alternatively, with CuCl2/CuO, but not with HgCl2/CdCO3 or AgNO3/AgO. Between FeCl3/CuO and CuCl2/CuO, the latter exhibited a shorter reaction time; thus, we focused on

Synthesis of Novel Basic Skeletons Derived from Naltrexone Fig. 5 Structures of acetal 11 and ortho ester 12

O

OH OMe

N

O OH 11

OH N

13

OMe OMe OMe OH O OMe

193

N

OMe

OH O

OMe

O OMe OMe

OMe

S S

N

OH

OO 14

OMe

12

OH N

OMe OMe

O 15

OMe

Fig. 6 Structures of compound 13–15

CuCl2/CuO in our subsequent optimization efforts. Next, we examined the effects of varying the amount of catalyst. When the level of CuCl2 was less than 0.25 equiv., no ortho ester 12 was obtained. However, when 1 equiv. of CuCl2 was used, good yields were obtained. When less than 0.5 equiv. of CuO was used, a remarkably low yield was obtained, and varying the ratio of CuCl2 to CuO led to the synthesis of other novel compounds 13–15 in addition to 12 (Fig. 6). The best yield of 12 (78%) was obtained when CuCl2 and CuO were included in equal amounts (1 equiv.). Substitution of K2CO3 for CuO resulted in a high yield of 12 (80%), which was the only product. In contrast, when CSA was substituted for CuO, little or no 12 was produced; however, this produced a good yield of 11 (65%) and a small amount of the cyclic acetal 8d (18%). In summary, the substitution of K2CO3 for CuO was a good approach for obtaining 12 without the related compounds 8d, 11, 13–15, and 19; substitution of CSA for CuO was a good approach for obtaining the dimethylacetal 11. We proposed that the synthesis of 12 proceeded via the mechanistic sequence depicted in Scheme 3. A key step in this sequence was the abstraction of the proton in the 20 -position by the alkoxide ion derived from the 14-hydroxide of the dithiane derivative 10. Previous reports have described the participation of this 14-OH group in the formation of naltrexone derivatives [13]. In our scheme, the 14-OH group of naltrexone (1) is activated intramolecularly by the lone electron pair on the 17-nitrogen; then the resulting 14-OH extracts the axial hydrogen from the 7-position of naltrexone (1). This facilitates enolization, and enol acetate is produced with Ac2O in pyridine at room temperature (Scheme 1). Consistent with this proposal, we found that the key intermediate 15 was isolated

194

H. Nagase and H. Fujii CuCl2 OH

O OH

N S

H

CuCl H S

O

S

N

10

H

O S

N

2'

OMe

CuCl2

S O OH

S O OH

OMe

OMe ClCu

S

O

O S

N

S S

N

OH

15

OH

O

O

OMe

OMe

CuCl S

MeOH

CuCl S

S

O

O

N

CuCl S

OMe S

N

O CuCl2

OH

N

OMe S CuCl

OH

OH

O

O

O

OMe

OMe

OMe

MeOH OMe

O N

OH

12

O OMe

Scheme 3 Proposed reaction mechanism for formation of the ortho ester 12

and readily hydrolyzed to 12 under the same conditions that enabled the acetal exchange reaction in 10. As a further test, we attempted to convert the 14-OH group to hydrogen in 10, to confirm that the 14-OH group participated in the abstraction of the 20 -hydrogen of dithiane 10. Compound 16 was synthesized from naltrexone methyl ether [22] and then subjected to the same acetal exchange reaction as above. Under the same conditions, acetal 18 was obtained instead of the objective ortho ester (Scheme 4), consistent with our hypothesis that the 14-OH group participated in the ortho ester formation. Subsequently, the obtained ortho ester 12 was readily hydrolyzed with acid to give ester 19 (Scheme 5).

Synthesis of Novel Basic Skeletons Derived from Naltrexone H

H

N

N a)

O O

16

17

H

S O OH

OMe

195

OMe

N S

b)

O OH

OMe

18

OMe

OMe

Scheme 4 Acetal exchange reaction of the 14-H compound 16. Reagents and conditions: (a) n-BuLi, 1,3-dithiane, DME, 70 C, 49%; (b) CuCl2·2H2O, CuO, CH(OMe)3, MeOH, 50 C, 63%

O

OMe OMe

N OH

OH a)

N

COOMe O OH

O 12

OMe

19

OMe

Scheme 5 Hydrolysis reaction of ortho ester 12. Reagents and conditions: (a) AcOH, 80 C, 67%

3 Synthesis of Opioid Ligands with Oxabicyclo[2.2.2]octane and Oxabicyclo[2.2.1]heptane Skeletons [23] As mentioned above, we synthesized the novel ortho ester 12 with an oxabicyclo [2.2.2]octane skeleton. However, this compound was not stable. The facile opening of the ether ring produced the corresponding hydroxy ester 19 (Scheme 5). Therefore, we sought to synthesize a stable basic skeleton 20. In the course of these studies, we developed new synthetic methods for an oxabicyclo[2.2.2]octane skeleton 20 and an oxabicyclo[2.2.1]heptane skeleton 21 (Fig. 7). The 6-keto group of naltrexone (1) is accessible to nucleophilic attack from the b-side to provide 6a-alcohol derivatives. Therefore, we attempted to synthesize a 6a-epoxide derivative 24 of naltrexone with a stable sulfur ylide derived from trimethylsulfoxonium iodide [24]. The 6a-epoxide 24 was expected to convert to the objective oxabicyclo[2.2.2]octane derivative 20. Instead, the 6b-epoxide 23 was obtained in 67% yield, but not the objective 6a-epoxide 24 (Scheme 6). The structure of 23 was determined by X-ray crystallographic analysis (Fig. 8). Although nucleophilic attack occurred on the b-side of naltrexone methyl ether (22), we postulated that the kinetically formed a-hydroxy intermediate A had returned to the original ketone (22) due to the strong dipole-dipole interaction between the 4,5-epoxy ring and the resulting hydroxide ion (Scheme 7). Ultimately, nucleophilic attack of the reagent occurred on the a-side due to a reversible reaction with the stable ylide to afford the stable b-alcohol B without the dipole-dipole interaction (Scheme 7). If this postulation was correct, the 4,5-epoxy open-ring compound 25 [25] could yield the objective 6a-epoxide 26 by reacting with the stable ylide. Although

196


O

N

OH

N OH O

20

OH

OH

21

OMe

Fig. 7 Structures of oxabicyclo[2.2.2]octane 20 and oxabicyclo[2.2.1]heptane derivative 21

OH

OH

N

N O O 22

O

a) O

OMe

23

OMe

OH N O O 24

20

OMe

Scheme 6 Reaction of naltrexone methyl ether (22) with a stable sulfur ylide. Reagents and conditions: (a) trimethylsulfoxonium iodide, NaH, THF, 55 C to rt, 67%

Fig. 8 X-ray crystallographic analyses of 20 and 23

compound 25 has a 4-OH group and not a 4,5-epoxy ring, the 4-OH group may be flexible enough to move into a position that avoids a dipole-dipole interaction with the a-OH group at the 6 position. Thus, the 6a-epoxide 26 might be expected to convert to the objective compound 27. However, surprisingly, when the open-ring


197 OH

Me

O S

N path A OH

strong repulsion O

O O path B 22

24

O O

path A

N

Me

OMe

A

CH2S(CH3)2 path B

OH

OMe N

O Me S Me O O

23

OMe

B

Scheme 7 Reaction mechanism of naltrexone methyl ether (22) with stable sulfur ylide to afford 6b-epoxide 23

OH

O

N

OH

N O OH

a)

OH

OMe

OMe

25

21

O

OH N

N OH OH

O OH 26

OMe

27

OMe

Scheme 8 Reaction of 4-hydroxy morphinan 25 with stable sulfur ylide. Reagents and conditions: (a) trimethylsulfoxonium iodide, NaH, THF, 55 C to rt, 58%

compound 25 was treated with the stable ylide at 55 C, an unexpected product 21 was obtained in 58% yield (Scheme 8). To investigate the source of the unexpected cyclization, we combined a precursor 28 that did not have a 4-hydroxy group with the same stable ylide. This produced compound 29 with an oxabicyclo[2.2.1]heptane skeleton via the epoxide 30. Alternatively, the morphinan methyl ether 28 could also react with the stable sulfur ylide derived from trimethylsulfoxonium iodide at room temperature. The definitive intermediate, 6a-epoxide 30, was isolated, and then treated with NaH in DMF at 80 C to produce the objective bicyclic compound 29 (Scheme 9).

198

H. Nagase and H. Fujii OH

O

N

OH

N a)

O OMe 28

29

b)

OMe

c) O

OH N

N

OH

O

30

OMe

31

OH

Scheme 9 Reaction of morphinan 28 with stable sulfur ylide to produce oxabicyclo[2.2.1]heptane derivative 29. Reagents and conditions: (a) trimethylsulfoxonium iodide, NaH, THF, 55 C, 80%; (b) trimethylsulfoxonium iodide, NaH, THF, rt; (c) NaH, DMF 80 C, 61%

OH N

O

O OH

N

OMe 30

OMe 29

Scheme 10 Intramolecular cyclization reaction of epoxide 30 to produce 29

The cyclization of 30 may proceed via the boat conformation. The distance between the 14-OH group and the 6-quarternary carbon would be closer than the distance between the 14-OH group and methylene carbon of the a-epoxide; thus, cyclization might occur and give 29, as shown in Scheme 10. On the other hand, Baldwin’s rule could explain this selective cyclization. This reaction was also conducted with the unstable ylide derived from trimethylsulfonium iodide to provide an epoxide 30 in 65% yield. Then treatment with NaH in DMF at 80 C afforded the objective compound 29 (Scheme 11). On the other hand, when naltrexone methyl ether (22) was treated with the unstable ylide at 0 C in THF-DMSO, the initial objective product 31 was obtained in 62% concomitantly with the methyl ether derivative 32 (Scheme 12). In this case, once the unstable ylide attacked the b-side of the 6-ketone, the ylide cannot be displaced, because it is an irreversible reaction. The presence of the 4,5-epoxy ring might disturb the formation of an ideal boat form in the resulting compound 33. This may bring the 14-hydroxy group and methylene group of the

Synthesis of Novel Basic Skeletons Derived from Naltrexone OH

199

OH

N

O

O

OH

N

N a)

b)

O

OMe

OMe

28

OMe

30

29

Scheme 11 Reaction of morphinan 28 with unstable sulfur ylide to give 29. Reagents and conditions: (a) trimethylsulfonium iodide, NaH, THF-DMSO, rt, 65%; (b) NaH, DMF, 80 C, 61%

O

OH N

O

N

N

a)

OH

O O 22

O

OMe

OMe

31

OMe

+

O 32

OMe

Scheme 12 Reaction of naltrexone methyl ether (22) with unstable sulfur ylide to afford 31 and 32. Reagents and conditions: (a) trimethylsulfonium iodide, NaH, THF-DMSO, 0 C, 31: 62%, 32: 26%

CH2S(CH3)2 OH

S

N

Me

N O

O O 22

Me

OH

N

Me S Me

OH

O O strong repulsion OMe

OMe

O 33

OMe

O N OH O 31

OMe

Scheme 13 Reaction mechanism for obtaining 31

methyl sulfonium moiety into closer proximity than the corresponding 6-carbon of 33; this would produce the oxabicyclo[2.2.2]octane skeleton 31 (Scheme 13). Demethylation of 31 with 1-propanethiol and t-BuOK afforded 20 (Scheme 14). The structure of 20 was also confirmed by X-ray crystallographic analysis (Fig. 8).

200 Scheme 14 Demethylation reaction of 31 to give 20. Reagents and conditions: (a) t-BuOK, 1-PrSH, DMF, 150 C, 96%


O N

N OH

OH

a)

O 31

OMe

O 20

OH

4 Novel Cleavage Reaction of the C16–N17 Bond in Naltrexone Derivatives [26] The 17-substituent of morphinans is believed to influence its agonistic or antagonistic activity towards the opioid receptor [5]. For example, 17-methyl morphinans, morphine, and oxymorphone are agonists. On the other hand, 17-allyl and 17-cyclopropylmethyl (CPM) morphinans, naloxone and naltrexone (1) are antagonists [5]. Therefore, a dealkylation, especially demethylation reaction of 17-substituents has been widely used to synthesize morphinan derivatives that possess various 17-substituents [27–33]. However, the reaction with chloroformates has only been applied to 14-H-morphinans. Therefore, we were interested in the 17-dealkylation reaction in 14-OH-morphinans, like naltrexone (1). In the course of the study, we found a novel reaction that cleaved the C16–N17 bond in a naltrexone derivative 35 that produced oxazolidinone 36, which lacked a D-ring (Scheme 15). This novel cleavage reaction is interesting from the viewpoints of both organic chemistry and medicinal chemistry, because this reaction could open the door for the synthesis of morphinan derivatives without a piperidine ring (D-ring). This reaction may be useful for converting a partial m opioid receptor agonist, morphine1 [34–38] into a full agonist derivative. In this section, we describe a novel reaction that cleaves the C16–N17 bond in the naltrexone derivative 35 and produces the oxazolidinone derivative 36. We first attempted to dealkylate the CPM group in naltrexone derivative 35 with 1-chloroethyl chloroformate (ACE-Cl) under the original reaction conditions [33]; however, the reaction hardly proceeded. After extensive investigation, we found that the piperidine ring of naltrexone derivative 35 could be cleaved with ACE-Cl (9 equiv.) in pyridine to give, surprisingly, oxazolidinone 36 in 60% yield concomitantly with the starting material 35. However, that reaction did not produce the objective carbamate derivative 37 (Scheme 15, condition (a); Fig. 9 shows the structure of compound 37). Therefore, we focused on the abnormal reaction that

1

Although morphine is a classical m opioid receptor agonist, its intrinsic activity is known to be lower than that of a representative full m opioid receptor agonist, DAMGO. Moreover, recent reports showed that morphine was a partial agonist for the opioid m receptor in some pharmacological assays. See [34–38].


201 O

17 N

OH

O

16

N

O

D

a), b), c), or d)

OO 35

O

Cl

OO

OMe

36

OMe

Scheme 15 Novel C16-N17 cleavage reaction with ACE-Cl. Reagents and conditions: (a) ACECl (9 equiv.), pyridine, 100 C, 60%; (b) ACE-Cl (22 equiv.), pyridine, 100 C, 80%; (c) ACE-Cl (5.6 equiv.), K2CO3, TCE, reflux, 93%; (d) Troc-Cl (5.6 equiv.), K2CO3, TCE, reflux, 70% O OH O Cl

N

O O

O

N

O

OO

37

OMe

OO

38

OMe

Fig. 9 Structures of carbamate 37 and the oxazolidinium intermediate 38

caused cleavage of the C16–N17 bond and produced oxazolidinone 36; we attempted to improve the yield of compound 36, and we also examined the reaction mechanism. We developed the working hypothesis that, first, a reaction intermediate, oxazolidinium 38 (Figs. 9 and 10) would be formed, and then chloride ion would attack C16. Sufficient chloride ions in the reaction system could facilitate an attack on C16 to cleave the C16–N17 bond. Insufficient free chloride ions would cause the cleavage reaction to proceed too slowly, and the remaining intermediate 38 would be hydrolyzed to give the starting material 35 during workup. Although pyridine might neutralize any HCl derived from the reaction of 35 with ACE-Cl, the resulting pyridinium chloride may limit the available free chloride ion in solution, because pyridine is a weak base and pyridinium chloride cannot dissociate effectively into free chloride ion and pyridine. On the basis of the above working hypothesis, we used a large excess of ACE-Cl (22 equiv.) in pyridine to facilitate production of the objective compound 36 (80% yield) (Scheme 15, condition (b)). We next attempted to use K2CO3 as a base. The K2CO3 reacts with HCl to produce KCl and H2CO3, which will decompose into CO2 and water. Thus, the starting naltrexone derivative 35 was treated with ACE-Cl (5.6 equiv.) in the presence of K2CO3 (9 equiv.) in 1,1,2,2-tetrachloroethane (TCE). We obtained the objective oxazolidinone 36 in 93% yield (Scheme 15, condition (c)). Finally, the reaction of 35 with 2,2,2-trichloroethyl chloroformate (Troc-Cl) also afforded 36 in 70% yield (Scheme 15, condition (d)). Next, we examined how the C16–N17 bond cleavage could occur without removing the CPM group. Figure 10 shows the respective intermediates 38

202


a O

b

O

c O

O CPM

14 17N 9

Cl

14 9 17N

16O

Cl O

OMe

N C9

OMe

38

O

C16

O

38

e

d

H

O Cl

14 17N 9

O

C16 16 O

OMe Cl

O

C9 N

O

CPM

39

Fig. 10 The postulated reaction mechanisms for bond cleavage in the respective intermediates 38 (a, b) and 39 (d) of 14-OH and 14-H-morphinan, with their Newman projection formulas (c, e). In intermediate 38, the CPM–N17 bond appears to be nearly orthogonal to the p-orbital (green) of the carbonyl group (a, c). However, the C16–N17 bond appears to be parallel to the p-orbital (green) (b, c). The CPM–N17 bond in intermediate 39 could be parallel to the p-orbital (green) (d, e)

(Fig. 10a, b) and 39 (Fig. 10d) of 14-OH and 14-H-morphinan with their Newman projection formulas (Fig. 10c, e). The figure includes a postulated reaction mechanism for bond cleavage. In Fig. 10a, the carbamate moiety of intermediate 38 would be fixed by the rigid ring structure. Therefore, the s-bond between the 17-nitrogen and the CPM group in 38 would be fixed nearly orthogonal to the p-orbital of the carbonyl group in the oxazolidinone ring; thus, it may not be conjugated with the carbonyl group of the oxazolidinone ring and, consequently, may not be activated by the carbonyl group. In contrast, the C16–N17 bond of 38 would be parallel to the p-orbital of the carbonyl group in the oxazolidinone ring (Fig. 10b). Thus, the s-orbital of the C16–N17 bond could be effectively conjugated with the carbonyl p-orbital, which would accelerate the cleavage of the C16–N17 bond in the reaction with a chloride ion. As a result, the C16–N17 bond would be stereoelectronically preferred for cleavage compared with the bond between the 17-nitrogen and the CPM group. The Newman projection formula of intermediate 38 (Fig. 10c) indicates the definitive difference between the position of the carbonyl p-orbital relative to the CPM–N17 bond and the C16–N17 bond. On the other hand, in intermediate 39 of 14-H-morphinan, the CPM group would be preferentially cleaved, because the N–C bond of the carbamate moiety could rotate freely and the s-bond between the 17-nitrogen and the CPM group could be activated (Fig. 10d, e).


203 O

Me N

OH

O Me N

O OO

40

OMe

O

a)

OO

41

OMe

Scheme 16 Reaction of 16,17-seco-naltrexone derivative 40 with Troc-Cl. Reagents and conditions: (a) Troc-Cl (10 equiv.), K2CO3, TCE, reflux, 65% Fig. 11 Structure of oxazolizinium intermediate 42

O Me N

O O OO

42

OMe

The reaction of acyclic amine 40 with Troc-Cl afforded the oxazolidinone derivative 41 without a CPM group in 65% yield (Scheme 16). In this case, the Me group is less hindered than the CPM group; therefore, Me group was predicted to be preferred for attack in a bimolecular nucleophilic substitution reaction. Nevertheless, the CPM group was preferentially removed over the Me group to give oxazolidinone 41. The conformation of reaction intermediate 42 (Fig. 11), was more flexible than that of 38; thus, both the N17–Me and N17–CPM bonds could be oriented parallel to the carbonyl p-orbital for activation. Therefore, the stereoelectronic effect could not explain the preferential cleavage of the CPM group. Alternatively, it may be explained by postulating the extreme stability of the cyclopropylcarbinyl cation [39–42]. Generally speaking, CPM tosylate can be solvolyzed 106 times faster than isobutyl tosylate because the tosylate ion could be solvolitically eliminated from the CPM tosylate to afford a stable cyclopropylcarbinyl cation [41]. The polarization of the CPM–N17 bond, probably due to the stability of the tentatively formed cyclopropylcarbinyl cation, may give the carbon of the CPM group a partial positive charge [43, 44] and, as a result, a nucleophile could easily attack on the carbon of the CPM group.

5 A Double Decarboxylation Reaction of an Oxazolidinone and a Carboxylic Acid: Its Application to the Synthesis of a New Opioid Lead Compound [45] In the above section, we described the C16–N17 bond cleavage reaction of naltrexone derivative 35 that gave oxazolidinone derivative 36 without a D-ring (Scheme 15). This cleavage reaction prompted us to reinvestigate the Beckett-Casy

204


opioid receptor – ligand binding model [46–48], using 4,5-epoxymorphinan derivatives cleaved in their D-rings [49]. In the course of the investigation, we found a novel double decarboxylation reaction. One of the resulting derivatives showed stronger affinity for the m receptor than morphine, the classical and clinically applied m agonist. In this section, we describe a novel double decarboxylation reaction and its application to the synthesis of opioid derivatives with a novel skeleton. Naltrexone acetal 35 was treated with ACE-Cl in the presence of K2CO3 in TCE to give oxazolidinone chloride 36 (Scheme 15) [26], followed by treatment with NaI to afford iodide 43 in 94% yield (Scheme 17). Ozonolysis of compound 44, prepared from iodide 43 by an elimination reaction, gave aldehyde 45. The resulting aldehyde 45 was oxidized with NaClO2 (Pinnick oxidation) [50] to afford carboxylic acid 46. Surprisingly, when carboxylic acid 46 was treated with K2CO3 in DMF, two carbon dioxide units (a carboxyl group and an oxazolidinone

O

O O

O

N

O

Cl

N a)

OO

36

O

I

OMe

43

O

b)

OO OMe

O O

N

O

c)

N H

O O O

OO

d)

OO

OMe

45

44

OMe

O N HO

O O

H N

O OO

46

OMe

O

e)

OO

47

OMe

Scheme 17 Elimination of the C15, C16 moiety of oxazolidinone chloride 36. Reagents and conditions: (a) NaI, DMF, 130 C, 94%; (b) t-BuOK, THF, 0 C, 99%; (c) O3, then Me2S, CH2Cl2, 78 C, 86%; (d) NaClO2, NaH2PO4, 2-methyl-2-butene, t-BuOH/H2O, rt, 84%; (e) K2CO3, DMF, 60 C, 93%


205

Fig. 12 ORTEP plot of 47

unit) were simultaneously eliminated to give olefin 47 in 93% yield. The structure of compound 47 was determined by X-ray crystallography (Fig. 12). The double decarboxylation of 46 was assumed to proceed via a four-membered ring intermediate C (b-lactone; Scheme 18)2 [51–58]. However, the treatment of compound 48, which lacks a carboxyl group, with K2CO3 in MeOH and water also produced the same decarboxylated compound 47 in 87% yield (Scheme 19), while the same treatment in DMF furnished the recovery of compound 48. This result may suggest an E2 or E1cB elimination mechanism3 rather than a mechanism that proceeds via a b-lactone. The E2 elimination usually proceeds via an antiperiplanar transition state. However, the C–O bond of the oxazolidinone ring lies in synclinal conformations with the carboxyl group in 46 and the hydrogen in 48. This suggested that the elimination reaction would not be likely to occur via an E2 fashion (some examples of syn eliminations have been described for molecules in which b-hydrogen and a leaving group could not achieve an antiperiplanar conformation; [62])4. Therefore, the double decarboxylation reaction was more likely to proceed via the E1cB mechanism. In that scenario, a carbanion D (Fig. 13), arising from the first decarboxylation of compound 46 or the deprotonation in compound 48 by K2CO3, might be stabilized by the phenyl ring and/or the carbonyl group in the oxazolidinone ring (Fig. 13). The decarboxylation of oxazolidinones 46 and 48 is seemingly similar to removal of the well-known amino protective group, the Fmoc group. Fmoc

2

The thermal decarboxylation of a b-lactone has been widely used for preparation of an olefin. See [51, 52]. Recent examples of the thermal decarboxylation of b-lactones are described in [53–57]. 3 Olefins have been prepared by elimination reactions, in which oxazolidinone functioned as a leaving group. See [59–61]. 4 In cyclic systems, the extent of anti and syn elimination depends on ring size. Cyclohexyl systems have a very strong preference for anti elimination. See [62, 63].

206


O

O O N O

O

O

N

O

O

O

O

OO

OO

OMe

46

OMe

O

O O

O

O

N

N O O

O

O

OO

O OO

OMe

O N

OMe

O O

H N

O

O

O

OO

OO

OMe

OMe

C

47

Scheme 18 Proposed decarboxylation mechanism of 46 via the four-membered ring intermediate C (b-lactone) O O N

O

H

H N

O

a)

OO

OO

OMe 48

OMe 47

Scheme 19 Decarboxylation of oxazolidinone 48. Reagents and conditions: (a) K2CO3, MeOH/ H2O, 80 C, 87% O O N

O OO

Fig. 13 The carbonyl group in the oxazolidinone ring may stabilize carbanion D

D

OMe

Synthesis of Novel Basic Skeletons Derived from Naltrexone Troc OAc 9a' N H

O

a)

207 H N

O

OO

49

OO

OMe

47

OMe

O O N

O

H b)

OO OMe 48

Scheme 20 Elimination reaction of compound 49. Reagents and conditions: (a) K2CO3, MeOH/ H2O, 80 C, 80%; (b) K2CO3, MeOH/H2O, rt, 94%

deprotection occurs through cleavage by a weak base, like an amine, via elimination and simultaneous decarboxylation [64]. Compound 47 was also obtained in 80% yield when 9a0 -acetyl-N-Troc derivative 49 was treated with K2CO3 in MeOH and water at 80 C (Scheme 20). It was proposed that compound 47 was produced by decarboxylation of 48 (Scheme 21, path A), because the treatment of compound 49 with K2CO3 at room temperature gave oxazolidinone 48 (Scheme 20); then 48 could be converted into compound 47 by the same treatment at 80 C (Scheme 19). An alternative route for the production of 47 could be via the sequential hydrolysis of carbamate and elimination of acetate (Scheme 21, path B). However, path B can be ruled out by the following results. First, hydrolysis of the carbamate required harsher reaction conditions and longer reaction time [26]. Second, when 9a0 -acetate 50 was hydrolyzed under basic conditions, only compound 52 was obtained (Scheme 22). Finally, treatment of compound 49 with Zn in AcOH afforded acetamide 54 by migration of the acetyl group (Scheme 23), but acetamide 54 was not converted into olefin 47 with basic treatment. Taken together, these results strongly suggest that compound 47 could be obtained via oxazolidinone 48 (path A), but not by path B (Scheme 21). Thus, the oxazolidinone structure may play an important role in the second decarboxylation reaction. Moreover, treatment of acyclic carbamate 51 with K2CO3 produced only compound 52 by hydrolysis (Scheme 22). This result supported the importance of the cyclic carbamate, oxazolidinone structure, in the second decarboxylation reaction. Compound 47 has a novel structure that lacks the D-ring in the 4,5-epoxymorphinan skeleton. Therefore, we next evaluated the affinity of 15-16 nornaltrexone derivative 57, derived from 47, for the opioid receptor. Compound 57 was prepared from 47 as shown in Scheme 24. The secondary amino group in olefin 47 was

208

H. Nagase and H. Fujii OH Me

Cl3C O

O

O

O

O

O

N

O

H

path A

N

O

H

OO OMe

49

Cl3C HO

OO

48

OMe

Me O

O

O

O

N

O

H N

O

path B

OO

OO

OMe

OMe

49

47

Scheme 21 Proposed decarboxylation mechanisms of 49 via oxazolidinone 48 (path A) or via sequential hydrolysis of carbamate and elimination of acetate (path B)

Me N

OR

Me N

9a'

O

H OO

OMe 50 (R = Ac) 51 (R = CONH2)

a)

OH O

H OO

52

OMe

Scheme 22 Hydrolysis of 9a0 -acetate 50 and 9a0 -carbamate 51. Reagents and conditions: (a) K2CO3, MeOH/H2O, 80 C, quant. from 50, 69% from 51

protected with a Cbz group followed by oxidation with mCPBA and subsequent hydrogenation of 55 to provide compound 56. The stereochemistries of compounds 55 and 56 were determined by 2D NMR analyses of their analogs. Compound 56 was converted into compound 52 by reductive alkylation, followed by acidic deprotection of the acetal and subsequent demethylation with BBr3 to produce the objective compound 57. In the competitive binding assay, compound 57 showed stronger binding affinity (Ki ¼ 1.17 nM) for the m receptor than morphine (Ki ¼ 1.98 nM). This indicated that compound 57 could be a new lead for designing receptor type-selective opioid ligands.


Troc N

OAc

OAc

H N

O

H

209

O

H

a)

OO

OO

OMe

49

OMe

53

Ac N

OH O

H OO

54

OMe

Scheme 23 Removal of the Troc protective group and migration of the acetyl group. Reagents and conditions: (a) Zn, AcOH, rt, 80%

H N

O

Cbz OH N

O

a), b)

OO OMe

OMe

47

H N

55

OH O

H

d)

Me N

OH O

H

OO

56

Me N

c)

OO

e), f)

OO

OMe

52

OMe

OH H O

57

O

OH

Scheme 24 Synthesis of 15–16 nornaltrexone derivative 57. Reagents and conditions: (a) Cbz-Cl, proton-sponge, CH2Cl2, rt; (b) mCPBA, CH2Cl2, 0 C, 68% from 47; (c) H2, 10% Pd/C, MeOH, rt, 69%; (d) (CH2O)n, NaBH3CN, AcOH, rt, 88%; (e) 2 M HCl, MeOH, reflux, quant.; (f) BBr3, CH2Cl2, rt, 19%

210


6 Synthesis of a Stable Iminium Salt and Propellane Derivatives [43] The 4,5-epoxymorphinan skeleton is believed to influence the intrinsic activity of the opioid receptor [10, 11, 65], and it is thought to contribute to three points of association between the drug molecule and the receptor site. These include an ionic interaction with the 17-nitrogen, a pp interaction with the phenol ring, and a hydrogen bond between the phenol hydroxy and the opioid receptor [46, 47]. However, the role of the 14-OH group in receptor binding has not been clarified. We attempted to remove the 14-OH group in order to compare the pharmacological profiles of 14-OH derivatives and 14-H variants. In the course of these investigations, we obtained iminium salt 59, which has a propellane skeleton. Surprisingly, the iminium salt 59 was stable and could be purified by silica gel column chromatography. In this section, we describe the synthesis of this stable iminium ion bearing a propellane skeleton. Morphinan derivative 58a, which was synthesized from naltrexone (1) [25, 66, 67], was dehydrated with SOCl2 in pyridine to afford the dehydrated product 60 and an abnormal rearrangement product 59 (iminium salt) (Scheme 25). The reaction mixture was separated by silica gel column chromatography with a CHCl3/MeOH eluent to isolate pure iminium ion 59 in 28% yield, together with the dehydrated product 60 in 71% yield. In 1H NMR analysis, a vinyl proton of iminium chloride 59 was observed at 10.13 ppm; this suggested that 59 was an iminium compound, not an a-chloroamine. In contrast, when 4,5-epoxymorphinan 35 was subjected to the same reaction conditions, only the dehydrated product 61 was obtained (Scheme 25) [22]. We became interested in the abnormal stability of the iminium salt and attempted to improve its yield. A plausible mechanism for the rearrangement reaction that produced the iminium chloride 59 is shown in Scheme 26. The lone electron pair of the 17-nitrogen might play a crucial role in facilitating the rearrangement necessary for obtaining 59. We hypothesized that a proton acquired during this reaction could protonate the 17-nitrogen, and interrupt the rearrangement reaction. Conversely, we postulated that reaction conditions in the absence of protons would facilitate the rearrangement and promote the production of the iminium salt 59. After extensive investigation of our working hypothesis, the reaction of morphinan 58a with MsCl and NaH provided the objective iminium 63a in 93% yield without the dehydrated product 60 (Table 1, entry 1). The rearrangement reactions of morphinans 58a–e with various 17-substituents produced moderate to excellent yields, except in the case of morphinan 58f. The morphinan 58f had a strong electron-withdrawing CF3CH2 group [68] at the 17position, and we only recovered the starting material 58f (entry 6). These results suggested that the 14-OH group may not be directly mesylated. Instead, the results shown in Table 1 could be reasonably explained by hypothesizing that the lone electron pair on the 17-nitrogen initially attacked the sulfene prepared in situ, and this was followed by migration of the Ms group from the 17-nitrogen to the 14-OH


17 N

OH 14

211

Cl O 5

N

O

N

O

a)

O

+

4 XO

XO

MeO

OMe 58a (X = -H, -H) 35 (X = -O-)

OMe 60 (X = -H, -H) 61 (X = -O-)

59

Scheme 25 Dehydration of morphinan 58a and 4,5-epoxymorphinan 35. Reagents and conditions: (a) SOCl2, py

OH N

O

O S

N

O

O N

O

O

58a

Cl Cl

O

O

OMe

62

OMe

MeO

59

Scheme 26 A plausible mechanism for the rearrangement of morphinan derivative 58a to produce the iminium salt 59

Table 1 Substituent effect on rearrangement reactiona MeSO3

OH R N

O

O

R N

O

O OMe

MeO

58a-f

Entry Morphinan Product R 1 58a 63a CPM 2 58b 63b i-Bu 3 58c 63c Me 4 58d 63d Et 5 58e 63e Allyl 6 58f 63f CF3CH2 a Morphinan 58 was treated with NaH and MsCl in THF b Including some amounts of inseparable impurities c Only the morphinan 58f was recovered

63a-f

Yield (%) 93 39 95b 93 92 0c

Reaction time (h) 1 2 1.25 1.25 3.5 31.5

212


Fig. 14 ORTEP plots of 63b and 68c

group.5 The strong electron-withdrawing CF3CH2 group [68] would drastically reduce the electron density on the 17-nitrogen and decrease its nucleophilicity, which would result in recovery of the starting material 58f. All the synthesized iminium 63 were purified by silica gel column chromatography. In 1H NMR analyses, a vinyl proton was observed at approximately 9.5 ppm for each iminium mesylate 63. Finally, compound 63b with a 17-i-Bu group was isolated as a crystalline product and its structure was determined by X-ray crystallography (Fig. 14). To the best of our knowledge, X-ray crystallographic analyses of iminium ions are very rare,6 except in the case of conjugated iminium ions.7 An example that most resembled iminiums 59 and 63 was the Fe(CO)3 complex of iminium tetrafluoroborate 64 (Fig. 15) [70, 83, 84]. In the case of iminium 64, anchimeric participation of the double bond derived from a homoallylic system and an inert counter anion, tetrafluoroborate, could stabilize the iminium ion. Moreover, the bulky Fe(CO)3 group may contribute to the stability of iminium ion 64. The other examples of nonconjugated iminium ions 65–67 (Fig. 15), whose structures were determined by X-ray crystallography, also had inert counter anions [69–71] and would be stabilized by an intramolecular interaction with an olefin moiety (iminium 65) [71] or by an intermolecular interaction with the phenyl groups of a counter anion (iminium 66). In contrast, iminium compounds 59 and 63 had neither an olefin moiety nor a Fe(CO)3 group to participate in the stabilization of the iminium ion. Furthermore, iminium ion 59 had a reactive counter anion, the chloride ion.

5

The reaction of 58a with the CPM group seemed to proceed most rapidly. This observation might be explained by considering the very stable cyclopropylcabinyl cation (see Sect. 4 and [39–42]). 6 For X-ray crystallography of nonconjugated iminium salts, see [69–71]. 7 For X-ray crystallography of conjugated iminium salts, see [72–82].

Synthesis of Novel Basic Skeletons Derived from Naltrexone Fig. 15 Iminium ions whose structures were determined by X-ray crystallography

BF4

213

(OC)3Fe CF3SO3

Me N OMe

HO MeO

NMe2

64

65

[Me2C=NHMe]+BPh4- (66a) +

[MeEtC=NHMe]+BPh4- (66d) [Me2C=NMe2]+ClO4- (67)

-

[Me2C=NMe2] BPh4 (66b) [Me2C=NHEt]+BPh4- (66c)

Fig. 16 Structures of compounds 63b and 68c

MeSO3 N1

C5

N

C6 C23

C11

O Me N

O

C13

C12

C7

MeO

O

HO 63b

68c

˚ ) in iminium The X-ray crystallography indicated that the length of C5–C6 (1.545 A 63b, which was parallel to the p-orbital of the N1–C23 double bond, was longer than that of the corresponding bond C11–C13 in the saturated compound 68c ˚ ), derived from iminium 63c (Fig. 16). Parallel bonds can easily overlap (1.532 A each other. This observation suggested that the s bond C5–C6 would interact with the p* bond of the iminium and that the electronic interaction (hyperconjugation) may play a crucial role in stabilizing the iminium ion. To investigate hyperconjugative stabilization of the iminium, we performed natural bond orbital (NBO) analysis [85] with the ab initio molecular orbital calculation at the HF/6-31G* level. This analysis used the second order perturbation theory of the Fock operator with the NBO basis. It indicated that there was substantial interaction from s (C5–C6) to p* (N1–C23) (interaction energy: 6.31 kcal/mol). The optimal bond ˚ longer than that of the corresponding bond in 68c.8 length of C5–C6 was 0.011 A Taken together, the stereoelectronic effects (hyperconjugation) attributed to the structures of iminiums 59 and 63b could play an important role in the extreme stabilization of these iminium ions.

8 ˚ longer than The NBO analysis showed that the optimal bond length of C6–C7 was also 0.007 A that of the corresponding bond in 68c, with an interaction energy between s (C6–C7) and p* (N1– C23) of 4.98 kcal/mol. However, in X-ray crystallography, the C6–C7 in 63b showed the same ˚ ) as the C13–C12 in 68c. This result suggested that the C5–C6 might also bond length (1.551 A partially participate in the stabilization of the iminium ion.

214


MeSO3

O N

MeO

O

O N

O

a)

MeO

63a

69

b)

CN N

O

MeO

H2N

O

O

N

O

c)

+ 69

MeO 70

71

Scheme 27 Reaction of iminium 63a. Reagents and conditions: (a) NaBH4, MeOH, 0 C, 81%; (b) NaCN, H2O, rt; (c) LiAlH4, THF, 0 C, 71: 26% from 63a, 69: 74% from 63a

MeSO3

C5 O N C23

O C7

Fig. 17 Structure of iminium 63a. The hyperconjugation is shown with dotted lines

MeO

63a

We next examined the reactivity of the stable iminium ion 63a (Scheme 27). When the iminium 63a was treated with NaOAc or NaN3, only the starting material was recovered. The reaction of 63a with n-BuLi or lithiated dithiane gave complex mixtures. The stronger basic nucleophiles, like n-BuLi and lithiated dithiane, may attack both the iminium carbon (C23) and the hyperconjugated carbons (C5) to give complex mixtures (Fig. 17). However, the reduction of iminium 63a with NaBH49 afforded the saturated propellane derivative 69. The reaction of iminium 63a with NaCN10 provided nitrile 70; then reduction of the nitrile 70 produced amine 71 in a

9

In the case of the Fe(CO)3 complex 64, the reduction was reported to take 20 min (see [84]). On the other hand, the reduction of 63a was completed within 5 min. 10 The reaction of the Fe(CO)3 complex 64 with NaCN was reported to proceed in acetone under reflux for 5 min (see [83]). In contrast, the reaction of 63a was completed at room temperature within 5 min.

Synthesis of Novel Basic Skeletons Derived from Naltrexone Fig. 18 Structures of propellane derivatives 68c and 68a and their Ki values for the m and k opioid receptors

O

Me N

HO

215

68c

Ki (μ) = 3.6 nM

O

N

HO

68a

Ki (κ) = 17.4 nM

mixture of diastereomers along with compound 69. Compound 69 may be obtained by reduction of the iminium derived from nitrile 70 via elimination of the cyanide ion. These observations suggested that the adduct 70 could be easily returned to the starting iminium ion 63a due to its extreme stability. In a preliminary pharmacological evaluation, saturated propellane 68c and its 17-CPM derivative 68a, which were synthesized from propellane 69, selectively bound to the m (Ki ¼ 3.6 nM) and k opioid receptors (Ki ¼ 17.4 nM), respectively (Fig. 18). These results suggested that propellane 68c, which was readily prepared from stable iminium ion 63c, might be a fundamental skeleton for novel opioid ligands. Birch et al. described synthesis of a propellane derivative from a Fe(CO)3 complex of the iminium ion 64 and its application as an opioid ligand. Nevertheless, they did not show the yield of iminium ion 64 with Fe(CO)3 or demonstrate the propellane derivative as an opioid ligand [83, 84]. Furthermore, their synthesis of the propellane derivative required flammable, toxic Fe(CO)5 as a crucial reagent and thebaine (narcotic) as a starting material, which is not readily available. In distinct contrast, our practical synthesis of a propellane derivative started with the readily available naltrexone and required no toxic reagents. Recently, researchers in China also published a description of propellane derivatives; however, their pharmacological data were limited to the indole derivatives [86].

7 Novel Synthesis of a 1,3,5-Trioxazatriquinan Skeleton Using a Nitrogen Clamp [87] As mentioned above, the twin drug 72 (Fig. 19) was also prepared from naltrexone (1) and showed e receptor antagonist activity [88]. In the course of the synthetic investigation of 72, we unexpectedly obtained a triplet drug 73 with a 1,3,5trioxazatriquinane skeleton (red color in Fig. 19). In this molecule, nitrogen plays an important role in the formation of the novel skeleton. Nitrogen is a trivalent atom that gathers the three molecular units through their keto moieties and fixes them in a precise rigid trimer. Thus, nitrogen acts like a three-pronged clamp, and we have termed this phenomenon a “nitrogen clamp”. The nitrogen clamp may be expected to apply to the general synthesis of triplet

216


N

OH MeO

RO

H O O

O O

O

N

N O

O

O

H OMe

OO

N

N O O

OH

H OMe

HO OR N

72a(R= H) 72b(R= Me)

73

Fig. 19 Structures of the twin drugs 72 and the triplet drug 73 OH

O

OH N

N

CHO OH O

OH O

OH N

4 steps O

9

O

OMe

1

N

OH OO OO HO OMe 74

8b, 9, 74

a)

72b

+

OMe

MeO

OH OH

8b

N OH

73

Scheme 28 Syntheses of acetal dimer 72b and trimer 73. Reagents and conditions: (a) CSA, toluene, reflux

drugs from various ketones that would show unique pharmacological effects. In this section, we describe a novel synthetic method for synthesizing triplet drugs with a rigid 1,3,5-trioxazatriquinane skeleton and a nitrogen clamp. We recently reported that the acetal dimer 72b, a precursor of the twin drug 72a, was obtained from a mixture of a-hydroxyaldehyde 9, hemiacetal 8b, and the hemiacetal dimer 74, derived from naltrexone (1) in four steps [88]. Examination of the synthesized products revealed that a novel trimer 73 with the 1,3,5-trioxazatriquinane skeleton was produced in 10% yield, concomitantly with acetal dimer 72b in 13% yield (Scheme 28). Initially, the nitrogen source was not clear but, after


217

a precise analysis, we determined that the nitrogen was derived from the solvent (saturated ammonia–CHCl3) that was used in the silica gel column chromatography for separation of the objective dimer 72b. The structure of trimer 73 was determined by X-ray crystallography (Fig. 20). We were interested in the novel structure of 73; therefore, we attempted to improve the yield of 73 and clarify the reaction mechanism. When the mixture of compounds 8b, 9, and 74 was treated with NH4Cl and AcONa in MeOH under reflux, the oxazoline dimer 75a (46%) and its isomer 75b (9%) were obtained (Scheme 29). Moreover, acetal 11, a precursor of compounds

Fig. 20 X-ray crystallography of trimer 73

OH

OH MeO

N

N OH S O OO * H

a)

8b, 9, 74

MeO

N

R N OH O OO * H

+

OMe

OMe HO

HO

N

75a

N

75b

OH OMe

N

O OH

OMe

b), c)

75a

+

d) 11

75b d)

OMe 73 (68%)

73 (30%) + 75a (30%)

Scheme 29 Synthesis of the trimer 73 via the dimers 75. Reagents and conditions: (a) NH4Cl, AcONa, MeOH, reflux, 75a: 46%, 75b: 9%; (b) 2 M HCl, reflux, (c) NH4Cl, AcONa, MeOH, reflux, 75a: 45%, 75b: 43%; (d) CSA, CHCl3, reflux

218


8b, 9, and 74, was hydrolyzed with 2 M HCl under reflux, and the resulting mixture (without purification) was also treated with NH4Cl and AcONa in MeOH under reflux. This produced a mixture of oxazoline 75a (45%) and isomer 75b (43%) (Scheme 29). The dimer 75b was treated with CSA in CHCl3 under reflux to give a mixture of dimer 75a (30%) and the trimer 73 (30%). On the other hand, the dimer 75a could be converted to the trimer 73 in 68% yield under the same conditions (Scheme 29). These observations supported the notion that the dimer 75a was the stable isomer and 75b was unstable one, but could be converted to the trimer 73 via its stable isomer 75a. Interestingly, X-ray analysis showed that the absolute configuration of the three newly formed asymmetric centers of the 1,3,5trioxazatriquinane moiety in trimer 73 were all in S configurations (Fig. 20). The selective formation of the asymmetric centers may be attributed to asymmetric induction by the natural naltrexone skeleton. We also assigned the configurations of the * positions in dimers 75a and 75b as S and R (Scheme 29), respectively, based on 2D NMR spectroscopy. Eventually, the R configuration isomer 75b would be the kinetically controlled product and the S isomer 75a would be the thermodynamically controlled product. Based on the above considerations, a series of reactions was performed without purification to convert acetal 11 into trimer 73 in 79% yield. Oxycodone (76) also provided trimer 77b in 6% total yield (Scheme 30). Next, we examined the application of this reaction to model compounds to investigate the generality of the reaction (Table 2). N-Benzylpiperidone (78) was converted to trimer 84 in 18% total yield. Acetophenone (79) was converted to a mixture of trimers 85a–85c in 35% total yield. Thioacetals 83a and 83b, obtained from 4-phenylcyclohexanone (80), were converted to trimers 86a (35%) and 86b

Me

N

OH RO H

OH

O O

Me N O O

OR

5 steps 6%

OMe

H

N

OO N O O

Me

OH

H OR

HO

76 N Me a)

77b (R = Me) 77a (R = H)

Scheme 30 Synthesis of the N-methyl trimer 77. Reagents and conditions: (a) BBr3, CH2Cl2, rt, 48%

2

Run 1

O

Me

Acetophenone (79)

O

N-Benzylpiperidone (78)

Bn N

Starting material

Table 2 General synthesis of trimers

R'

HO

Bn N S

82

S Me

S

81

OH S

Intermediate

R

O

H Ph

84

O

N H

Ph

Me

N Bn

H Me

N

Me H O

Ph

O

O H

Me Ph

85c 10%

H Ph

O

H

O

O

Me H

O

Ph

N

N

H

Ph

Bn

H

O

Bn N

R R'

85b 14%

Ph

O

H

Me

H

Me

R

O

Product

H R'

N

R' H

Me

O

O

R'

S

O

R

85a 11%

N

Ph H

O

Me

R

HO

S

35

(continued)

Total yield (%) 18

Synthesis of Novel Basic Skeletons Derived from Naltrexone 219

3

Run

O

4-Phenylcyclohexanone (80)

Ph

Starting material

Table 2 (continued)

R'

Ph

Ph

S

OH S

S

83b

83a

OH S

Intermediate

R

O R

HO R'

S S R

N

R' H

O

O

Ph

Ph

Ph

Ph

O

O O

O

H

R R'

O

O

86b 13%

N

86a 35%

N

Product

H R'

O

R

Ph

Ph

48

Total yield (%)

220 H. Nagase and H. Fujii


221

(13%), respectively. We also attempted to apply this reaction to the synthesis of a nonsymmetric trimer (Scheme 31). The dimer 87, prepared from 4-phenylcyclohexanone (80), was reacted with a-hydroxyaldehyde 93 (CSA/CHCl3, reflux) to produce the nonsymmetrical trimer 88 in 91% yield. The dimer 87 was also reacted with the glycolaldehyde dimer (94) (CSA/CHCl3, rt) to give the nonsymmetric trimer 89 in 98% yield. Furthermore, dimers 75a and 75b, obtained from a mixture of 8b, 9, and 74, were reacted with 96 (derived from haloperidol), the glycolaldehyde dimer (94), and a-hydroxyaldehyde 95 to produce the nonsymmetric trimers 90, 91, and 92 in yields of 23, 75, and 50%, respectively. The symmetrical trimer 77a (Scheme 30), obtained from oxycodone (76), showed potent analgesic action in the acetic acid writhing assay (ED50 ¼ 0.037 mg/kg). The antinociceptive effect was approximately 20 times more potent than that of morphine (ED50 ¼ 0.6 mg/kg). This strong analgesic potency may derive from the binding with oligomeric receptor sites [89, 90]. In conclusion, we have established the first general method for synthesizing a trimer with a 1,3,5-trioxazatriquinane skeleton fixed with a nitrogen clamp that can be applied to the synthesis of both symmetrical and nonsymmetrical trimers. Recent studies have reported that G-protein-coupled receptors (GPCRs) exist as dimers, but may be present as either homo- or hetero-dimers/oligomers. Although the existence of GPCR dimers/oligomers was predicted from early pharmacological and biochemical studies [89, 90], further evaluations of this phenomenon have been impeded by the lack of appropriate reagents [91]. Twin drugs that combined two structural components into a single molecule have been described in numerous domains of medicinal chemistry. Symmetrical twin drugs can bind simultaneously to symmetrical binding sites of a protein complex for increased activity. In contrast, Bn N O O

N

Ph

a) N

Ph

O OH

O 87

88

Ph Ph O

b)

O N

Ph

O Ph 89

Scheme 31 Syntheses of nonsymmetric trimers 88–92. Reagents and conditions: (a) 93, CSA, CHCl3, reflux, 91%; (b) 94, CSA, CHCl3, rt, 98%; (c) 96, CSA, CHCl3, reflux, 23% from mixture of 8b, 9, 74; (d) 94, CSA, CHCl3, rt, 75% from mixture of 8b, 9, 74; (e) 95, CSA, CHCl3, reflux, 50% from mixture of 8b, 9, 74

222


MeO

N OO H OMe

N OH O

HO

N

75a,b Cl

e)

c)

Me

OH N MeO H

F O

N

OH

d) O O

OO

N

N

H

H

O O

H OMe

OH OMe

MeO H OO O O

HO

HO

N N 90

H HO

O

N

O O

N

OO

92

OMe

H

H OMe

N

HO 91 OH OH

Bn N 93 O HO

Me N

OMe

95

OH

O 94

OH Cl

HO CHO N F

Scheme 31 (Continued)

CHO O OH

CHO

96

N

N H

OH OMe


223

nonsymmetrical twin drugs may bind to the individual relevant binding sites to provide dual actions. However, twin drugs can play only one role, either an increase in activity or dual actions. On the other hand, nonsymmetric triplet drugs with two identical moieties and one different moiety may exhibit both increased pharmacological activity and dual actions, because the two identical portions could bind identical receptor sites simultaneously, and the third portion could bind a different receptor site or enzyme. Moreover, a symmetrical triplet drug 77a showed 20 times stronger analgesic activity than the monomeric compound morphine. This suggested that symmetrical triplet drugs could provide extremely potent activities. Therefore, the thus obtained trimers with a rigid 1,3,5-trioxazatriquinane skeleton fixed with a nitrogen clamp may be a very useful tool for investigation of oligomeric receptors and enzymes.

8 New Rearrangement Reaction for the Synthesis of a New Opioid Ligand with an Oxabicyclo[3.2.1]octane Skeleton [92] In Sect. 7, we described a method for the synthesis of the rigid triplet drug 73 with a 1,3,5-trioxazatriquinane skeleton and three naltrexone units with 4,5-epoxy rings [87]. This synthetic method could be applied to three general ketones (N-benzylpiperidone (78), acetophenone (79), and 4-phenylcyclohexanone (80)) [87]. However, this method could not be applied to dimethyl acetal 98, which was synthesized from morphinan derivative 28 [23, 93] in two steps. When 98 was subjected to the same reaction conditions as those applied to the naltrexone derivative (4,5-epoxymorphinan), the monomer 102 was obtained in 21% yield instead of the objective trimer 101 (Scheme 32). The structure of the monomer was identified as the novel oxabicyclo[3.2.1]octane derivative 102 by 2D NMR, MS, and IR spectroscopy (Fig. 21). We were interested in the novel compound 102 and examined the reaction mechanism (Scheme 33). We hypothesized that a dimer 100, derived from a-hydroxyaldehyde 99, would be more flexible than 75, derived from naltrexone methyl ether (22). Thus, the 14-OH group may readily participate in the double bond of the oxazoline ring of 100 to produce the cyclic ether 103. Moreover, the oxonium intermediate 104 may also be more flexible than a structure with a 4,5epoxy ring; thus, the C6–C70 s-bond in 104 may be readily rearranged to the oxonium double bond to afford the rearrangement product 102. If our hypothesis was correct, the oxonium ion 990 , derived from dimethyl acetal 98, would rearrange more readily than oxazoline 100 to give 102. The bond between the 6-carbon and 60 -oxonium carbon in 990 can rotate freely around the axis; this would facilitate the approach of the 14-OH group and its overlap with the p orbital of the carbonyl carbon in 990 to produce the cyclic acetal intermediate 105. Then 105 could rearrange to 102 via the oxonium 106 (Scheme 34). Furthermore, we had shown that the three aforementioned general ketones (N-benzylpiperidone (78), acetophenone (79), and 4-phenylcyclohexanone (80)) without OH groups

224


OH

N O

28

OH

S

N

S

a)

OMe

N

OMe

b) OH

OH

OMe

OMe

97

OMe

98 OH

MeO

N

OH

N OH

N

CHO

c)

d)

O H

OH

OMe 99

HO

OMe

e)

e)

N O N

N

100

OH MeO

O

H O O

OMe 102 21% from 98

N

N

H OMe

O

OH

H OMe

HO N 101

Scheme 32 Synthesis of oxabicyclo[3.2.1]octane derivative 102. Reagents and conditions: (a) nBuLi, 1,3-dithiane, DME, 70 C, 69%; (b) CuCl2·2H2O, CSA, CH(OMe)3, MeOH, 50 C, 37%; (c) 2 M HCl, reflux; (d) NH4Cl, AcONa, MeOH, reflux,; (e) CSA, CHCl3, reflux O N

Fig. 21 Observed nuclear Overhauser effect (NOE) spectra of compound 102

O ca.2.7 ppm H NOE H 6.52 ppm OMe

were converted to the corresponding trimers. Therefore, the 14-OH group could be expected to participate in the rearrangement reaction. To test our hypothesis, dimethyl acetal 98 was treated with CSA in toluene under reflux to give the


225

H

OH

MeO

N

N OH O H

101

OMe 100

H O

HO

N

MeO

NH OH

N

N

O

OMe

OH

103 MeO O H2 N

N

OH

7' 6

N

O

102

OH OMe

104

Scheme 33 Reaction mechanism for producing compound 102

expected compound 102 in 90% yield (Scheme 35). In contrast, the dimethyl acetal 11 with a 4,5-epoxy ring could not be converted to the corresponding rearrangement product 108. The existence of the 4,5-epoxy ring in 109 may have fixed the morphinan skeleton and thus, prevented the 70 -carbon from rearranging to the oxonium carbon (Scheme 35). To evaluate the opioid receptor binding affinities of the rearranged compounds, compound 102 was demethylated with BBr3 in CH2Cl2 to produce phenol 107 in 87% yield (Scheme 35). Compound 107 showed strong affinity for both the m and k opioid receptors, comparable to that of naltrexone (1) (Table 3). We previously reported the synthesis of compound 110 with an oxabicyclo[2.2.1]heptanes skeleton and a C-ring in a fixed boat conformation, and the synthesis of its derivative (Fig. 22) [94]. At that time, we postulated that the potent k agonist TRK-820 also had a C-ring in the boat conformation [95, 96]. Subsequently, we showed that the amide derivative 110 had strong k agonist activity (Fig. 22) [94]. This result

226


OMe

14

N 98

H OMe

O

OH

N

H

6'

6

OH

OH OMe

99'

OMe

105

O

O

N

O

N OH

106

OMe

102

OMe

Scheme 34 Plausible mechanism for the rearrangement reaction for providing 102 from 98

OH

O

OMe

N

OMe OH

O

N a)

b)

102

OMe

OH

98

107

OH

O

OMe

N

O OH

OMe

a)

N

N OH

O

OMe 11

O

O

OMe 108

O

109

OMe

Scheme 35 Comparison between the reactivities of dimethyl acetals 98 and 11. Reagents and conditions: (a) CSA, toluene, reflux, 90%; (b) BBr3, CH2Cl2, rt, 87%

supported our prediction. Compound 107 also has a C-ring in the fixed boat conformation, i.e., a novel oxabicyclo[3.2.1]octane skeleton. Compound 111, which is the fundamental skeleton of 110, also had k type selectivity (m/k Ki ratio ¼ 1.42). Compound 107 showed a rather improved selectivity (m/k Ki ratio ¼ 1.70) over that of 111; however, 107 had reduced affinities for each opioid receptor type compared to 111. We expected to improve both the affinity and k selectivity of compound 107 by introducing a C6 side chain, similar to that present in TRK-820 [95, 96].


227

Table 3 Opioid receptor binding affinities of compound 107, compound 111, and naltrexone (1)a Compound Ki (k)b Ki (m)c Ki (d)d 107 0.233 nM 0.397 nM 6.84 nM 111 0.102 nM 0.145 nM 3.50 nM Naltrexone (1) 0.373 nM 0.335 nM 20.7 nM a Binding assay was carried out in duplicate using homogenate of guinea-pig brain (k: cerebellum, m and d: forebrain) b 3 [ H]U-69,593 was used c 3 [ H]DAMGO was used d 3 [ H]NTI was used

O O

O N H

N C

110

OH

N C

OH

111

OH

O H N

O

Me

N C

O

O OH TRK- 820

Fig. 22 Structure of TRK-820 with the C-ring fixed in boat conformation via an intramolecular interaction between the 14-OH and amide nitrogen; morphinan derivatives 110 and 111 with oxabicyclo[2.2.1]heptane skeletons

9 Novel Rearrangement Reaction for Converting a 6,14-Endoethanomorphinan Derivative to a Benzomorphan Derivative [97] As mentioned in Sect. 2, we hypothesized that the strong affinity of TAN-821 for the m opioid receptor may derive, in part, from the high lipophilicity conferred by the 7,8-ethylene bridge. Therefore, we attempted to modify the endoethenotetrahydrothebaine skeleton to improve the e selectivity of TAN-821 in vitro. The ethylene bridge of TAN-821 is on the C-ring [19]. We hypothesized that the substituent above the C ring contributed to the e receptor selectivity [19]. Based on this hypothesis, we designed a naltrexone derivative 113 with a 14-allyl group to which appropriate substituents could be introduced. We planned to synthesize 113 from the primary alcohol 112 by fragmentation (Scheme 36). In the course of our attempts to convert the alcohol 112 to the 14-allyl derivative 113, we found a

228


7

N

OH N

6

14

OMe

O O

O

112

OMe

113

OMe

Scheme 36 Planned synthesis of compound 113 from compound 112

Me

O

H

N Me

N

N

N

O

OH O

Me Me

O

OH TAN-821

114

OMe

OH Pentazocine

Fig. 23 Structures of TAN-821, rearrangement product 114, and pentazocine

R1

R2

HO R1 R2 N

N OMe

H+

O O

O

115

OMe

116

OMe

Scheme 37 Formation of the 14-alkenyl derivatives 116 from sec- or tert-alcohols 115 after treatment with acid

novel rearrangement reaction of the compound 112 that afforded the benzomorphan derivative 114, which can be converted to the representative benzomorphan, pentazocine (Fig. 23). We became interested in this novel reaction and attempted to clarify the reaction mechanism. The endoethano- or endoethenotetrahydrothebaine derivatives 115 with either a tertiary or secondary alcohol moiety were reported to undergo fragmentation under acidic conditions to give 14-alkenyl derivatives 116 (Scheme 37)11 [98–102]. 11

The fragmentation of a 7-acyl-6,14-endoethenotetrahydrothebaine derivative was also reported. See [99].


229

CO2Et

CO2Et

N

N OMe

a)

OMe

O

117

b)

O

OMe

118

112

OMe

Scheme 38 Preparation of primary alcohol 112 from ester 117. Reagents and conditions: (a) H2 (0.4 MPa), Pd/C, EtOH, 50 C, 93%; (b) LiAlH4, THF, 0 C to rt, 95% Me N OMe O

119

a), b), c)

112

OMe

d)

N

O O

114

OMe

e)

Scheme 39 Preparation of rearrangement product 114 from compound 112 or 119. Reagents and conditions: (a) Tf2O, CH2Cl2, rt; (b) washing with sat. NaHCO3 aq., (c) toluene, reflux, 91%; (d) Tf2O CH2Cl2, rt, then toluene, reflux, 77%; (e) TfOH, toluene, reflux, quant

We attempted to cleave the C6–C7 bond of alcohol 112, which possessed a primary alcohol, to afford a naltrexone derivative 113 with a 14-allyl group (Scheme 36). Alcohol 112 was synthesized from compound 117 [19] by hydrogenation and subsequent reduction of the ester group (Scheme 38). We treated 112 with Tf2O in CH2Cl2 at room temperature. After the reaction mixture was washed with a saturated NaHCO3 solution and concentrated under reduced pressure, the resulting residue was dissolved in toluene and refluxed under argon to produce the olefin 119 (the 7-methylidene derivative has been prepared by other methods; for example, a Hofmann degradation of a 7-dimethylaminomethyl derivative, or treatment of the tosylate of the corresponding primary alcohol with t-BuOK [103]) in 91% yield. On the other hand, when 112 was treated with Tf2O in CH2Cl2, but not washed with NaHCO3 solution, and then the concentrated reaction mixture was refluxed in toluene under argon, the unexpected rearrangement compound 114 was obtained in 77% yield (Scheme 39). The structure of lactone 114 was determined by IR, mass, and NMR spectra (1H, 13C, COSY, HSQC, and HMBC) (Fig. 24). We then attempted to clarify the reaction mechanism of this novel rearrangement. When the olefin 119 was treated with TfOH (2.2 equiv.) in toluene under reflux, the objective lactone 114 was obtained in quantitative yield (when the 7-methylidene derivative with a 6,14-etheno bridge was treated with perchloric acid, another product was obtained [104]). Next, treatment of the olefin 119 with a

230


Fig. 24 Structure determination of 114, 120, and 121, and observed HMBC

Me H H O

N H

114 H

O OMe H

Me

Me N

N

H H

H

O OH OMe

120

121

O OH OMe

Me Me N 119

N

a) O OH 120 b) 114

OMe

+

O OH 121

OMe

b) 114

Scheme 40 Preparation of 114 from 120 or 121. Reagents and conditions: (a) CSA, toluene, reflux, 120: 34%, 121: 35%; (b) TfOH, toluene, reflux, quant

weaker acid CSA in toluene under reflux gave ketones 120 and 121 (Scheme 40), whose structures were also determined by IR, mass, and NMR spectra (Fig. 24). Both ketones 120 and 121 were converted into lactone 114 after treatment with TfOH in toluene under reflux. Taken together, these outcomes indicated that the three compounds 119–121 are intermediates in the production of lactone 114. On the basis of the structures of the three isolated important intermediates 119– 121, we proposed a reaction mechanism for the production of lactone 114 (Scheme 41). In this mechanism, catalytic amounts of water or TfOH would play an important role. Oxonium 1200 may be obtained by an intramolecular aldol reaction of the intermediate 122. Compound 122 may be derived from olefin 119 by acidic hydration, subsequent fragmentation, and isomerization. Oxonium 1200 may undergo intramolecular cyclization to give acetal 123; then fragmentation


231

H OH Me N

N

112 + Tf2O

OMe

119

O

OMe

OMe

OH Me

OH Me

Me N

N

N

121'

OMe OH

OMe OH

OMe

OMe

H

HO

OMe

OMe OH

122'

N

OMe

OH Me

Me

Me N

OMe

O

N OMe

OMe OH

O OMe

OH

OMe

122

OMe

H

Me

Me

Me N

N

N

OMe

OMe OMe OH

O OMe

O

OMe

123

120' Me

OMe

Me

Me N

O

N

O

O OMe 124

114

Scheme 41 Reaction mechanism to lactone 114

N

OMe

O

O

OMe

OMe

232


and subsequent isomerization would provide lactone 114. Alternatively, oxonium 1200 may be hydrated and undergo fragmentation to afford carboxylic acid; then esterification of the resulting carboxylic acid may lead to the production of lactone 114. However, we believe the former route would predominate because intramolecular reactions generally proceed faster than intermolecular reactions. Oxonium 1210 may be obtained from 1220 (regioisomer of 122) by an intramolecular aldol reaction. Both oxoniums 1200 and 1210 would be converted to lactone 114 due to the equilibrium between 1200 and 1210 under the reaction conditions. The equilibrium was confirmed by the observation that treatment of either pure compound 120 or 121 in the presence of CSA under reflux gave mixture of ketones 120 and 121. Moreover, although oxonium 1210 could be readily converted to lactone 114 via the isomeric oxonium 1200 , lactone 124 was not detected at all. This was surprising, because lactone 124 was expected be obtained directly from 1210 by a reaction sequence comparable to that which converts 1200 to 114 (Scheme 41) Thus, lactones 114 and 124 may have very different reactivities. The olefin moiety and the carbonyl group in lactone 124 may be sufficiently close in space that the reaction rapidly reversed to give ketone 121 (Fig. 25). That is, the equilibrium for the formation of lactone 124 would favor the reversal to ketone 121. On the other hand, the distance between the olefin moiety and the carbonyl group in lactone 114 may be sufficiently large that the reverse reaction may not occur. Therefore, formation of lactone 114 may be irreversible (Fig. 25). As a result, only lactone 114 would be obtained in high yield, regardless of the ratio of 120 and 121 at equilibrium. The structure of the rearranged product 114 is very similar to that of the benzomorphan, like pentazocine (Fig. 23) [105, 106], a useful analgesic. Although several papers have previously described the total synthesis of pentazocine, many

Me N

Me N

O

O OH 121

O

OMe

Me N

OMe

124 short distance Me

N

O

O O

O long distance

MeO

MeO 114

Fig. 25 Backside views of lactone 114 and 124

124


233

were syntheses of a racemic form, but there are several exceptions12 [107–115, 117–121]. Lactone 114 is a chiral compound with the desirable absolute configuration because it was derived from the commercially available semi-synthetic opioid ()-naltrexone via alcohol 112. Therefore, the conversion of lactone 114 to the pharmacologically active enantiomer ()-pentazocine would be expected [116]. We are currently investigating the conversion of 114 to pentazocine and plan to publish our report in the future.

10

Concluding Remarks

In this chapter we have described eight interesting reactions using naltrexone derivatives. Although almost all these reactions are characteristic of naltrexone derivatives, they could lead to many novel skeletons and provide new interesting pharmacological data. Some new reactions found using naltrexone derivatives were expanded to general reactions. For example, the 6a-hydroxy-6-aldehyde derivation from naltrexone that led to the oxazoline dimer and the 1,3,5-trioxazatriquinane skeleton (triplet drug) was applied to general ketones. These ketones were converted to a-hydroxyaldehydes, followed by conversion to the dimers and trimers (Sect. 7). Furthermore, the reaction in which the 17-nitrogen moiety of naltrexone derivatives interacted with osmium tetroxide to give amide derivatives could apply to general tertiary amines [122]. The ring opening reaction of the cyclopropane ring in the 17-CPM group in naltrexone derivatives with PtO2 in HBr-MeOH to provide an i-Bu group could also be applied to general cyclopropylalkyl derivatives for the exclusive production of branched alkyl derivatives [44, 123]. Finally, there are many other interesting reactions and pharmacological data that could not be described in this chapter due to space limitations. We hope that interested readers will refer to the reference list below [19, 22, 49, 66, 88, 93, 94, 124–128]. Acknowledgments We acknowledge the Institute of Instrumental Analysis of Kitasato University, School of Pharmacy for its facilities. We also acknowledge the financial support from Grantin-Aid for Scientific Research and the Uehara Memorial Foundation and the Shorai Foundation for Science and Technology. We reused some content from [16] (Copyright (2006)), [23] (Copyright (2007)), [26] (Copyright (2008)), [43] (Copyright (2008)), [45] (Copyright (2010)), [87] (Copyright (2009)), [92] (Copyright (2009)), and [97] (Copyright (2009)) with permission from the American Chemical Society or Elsevier.

12

For synthesis of pentazocine, see [107–111]. For synthesis of (–)-pentazocine with optical resolution, see [112, 113]. For the asymmetric synthesis of (–)-pentazocine, see the asymmetric synthesis of (–)-metazocine (an N-methyl derivative of (–)-pentazocine) has also been reported, [114]; very recently, the asymmetric synthesis of (–)-9-epi-pentazocine was also reported [115]; it has been shown that (–)-pentazocine is more potent than its antipode [116].

234


References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

18.

19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38.

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Twin and Triplet Drugs in Opioid Research Hideaki Fujii

Abstract Twin and triplet drugs are defined as compounds that contain respectively two and three pharmacophore components exerting pharmacological effects in a molecule. The twin drug bearing the same pharmacophores is a “symmetrical twin drug”, whereas that possessing different pharmacophores is a “nonsymmetrical twin drug.” In general, the symmetrical twin drug is expected to produce more potent and/or selective pharmacological effects, whereas the nonsymmetrical twin drug is anticipated to show both pharmacological activities stemming from the individual pharmacophores (dual action). On the other hand, nonsymmetrical triplet drugs, which have two of the same pharmacophores and one different moiety, are expected to elicit both increased pharmacological action and dual action. The two identical portions could bind the same receptor sites simultaneously while the third portion could bind a different receptor site or enzyme. This review will mainly focus on the twin and triplet drugs with an evaluation of their in vivo pharmacological effects, and will also include a description of their pharmacology and synthesis. Keywords Dual action Increment of potency Receptor dimerization Twin drug Triplet drug Contents 1 2

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241 Twin Drugs Consisting of Two Opioid Pharmacophores . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243 2.1 m and k Pharmacophores . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243 2.2 d and k Pharmacophores . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251 2.3 m and d Pharmacophores . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255 2.4 Symmetrical Twin Drug . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257

H. Fujii School of Pharmacy, Kitasato University, 5-9-1, Shirokane, Minato-ku, Tokyo 108-8641, Japan e-mail: [email protected]

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3

Twin Drugs Consisting of Opioid and the Other Pharmacophores . . . . . . . . . . . . . . . . . . . . . 258 3.1 Opioid and Imidazoline Binding Site Pharmacophores . . . . . . . . . . . . . . . . . . . . . . . . . . . 259 3.2 Opioid and Substance P Pharmacophores . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 261 3.3 Opioid and Cholecystokinin Pharmacophores . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 262 3.4 Miscellaneous . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265 4 Triplet Drugs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 266 5 Conclusion and Perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 266 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 271

Abbreviations A ADME b-FNA BNTX BRET CBM CCK COS CPM CPP DAMGO DCC DML Dmt DPDPE EC50 ED50 EDC Emax GNTI GPI GTP HOBt 5-HT IBS IC50 i.c.v. i.m. Imax i.p. i.t.

Adenosine Absorption, distribution, metabolism, and excretion b-Funaltrexamine 7-Benzylidenenaltrexone Bioluminescence resonance energy transfer Cyclobutylmethyl Cholecystokinin CV-1 origin SV40 cell Cyclopropylmethyl Conditioned place preference [D-Ala2, N-Me-Phe4, Gly-ol]enkephalin Dicyclohexylcarbodiimide Designed multiple ligand 20 ,60 -Dimethyltyrosine Cyclic[D-Pen2, D-Pen5]enkephalin 50% Effective concentration 50% Effective dose 1-Ethyl-3-(3-dimethylaminopropyl)carbodiimide % Maximal stimulation Guanidinonaltrindole Guinea pig ileum Guanosine triphosphate N-Hydroxybenzotriazole 5-Hydroxytryptamine (serotonin) Imidazoline binding site 50% Inhibitory concentration Intracerebroventricular Intramuscular % Maximal inhibition Intraperitoneal or intraperitoneally Intrathecal

Twin and Triplet Drugs in Opioid Research

i.v. MAPK MS 3A NMM nor-BNI NSAID NTB NTI s.c. SP Tic

241

Intravenous Mitogen-activated protein kinase ˚ Molecular sieves 3A N-Methylmorpholine nor-Binaltorphimine Nonsteroidal anti-inflammatory drug Naltriben Naltrindole Subcutaneous Substance P 1,2,3,4-Tetrahydroisoquinoline-3-carboxylic acid

1 Introduction Twin drugs are defined as compounds that contain two components exerting pharmacological effects, that is, pharmacophores in a molecule. The twin drug having the same pharmacophores is a “symmetrical twin drug”, whereas one possessing two different pharmacophores is a “nonsymmetrical twin drug”, Although several terms have been used such as bivalent ligands, chimeric compounds, hybrid compounds, conjugated ligands, and so on, in this chapter the term “twin drugs” will mainly be used to describe these drugs. In general, the symmetrical twin drug is expected to produce more potent and/or selective pharmacological effects, whereas the nonsymmetrical twin drug is anticipated to show two different pharmacological activities arising from the individual pharmacophores (dual action) [1]. The report that an enkephalin analog induced clustering of opioid receptors on the surface of neuroblastoma cells [2] prompted Portoghese and his colleagues to design and to synthesize symmetric twin drugs 1–3 (Fig. 1) consisting of two 6-oxymorphamines (R ¼ Me) or 6-naltrexamines (R ¼ CPM (cyclopropylmethyl)) to investigate the receptor clustering [3, 4]. The investigation using the

R

HO

OH N

O 6

N O H OH

O

H N O

N H

H O N O

6

N OH

1: 6β, R = Me 2: 6α, R = Me 3: 6β, R = CPM (cyclopropylmethyl)

Fig. 1 Structures of twin drugs 1–3

R

242

H. Fujii OH

OH

N

OH N

N O H O OH HO nor-BNI (4)

N N O H OH NTI (5)

Fig. 2 Structures of nor-BNI (4) and NTI (5)

twin drugs led to the establishment of the “message-address” concept [5, 6], which is a useful guideline for designing selective ligands for an opioid receptor type, and contributed to the development of the first highly selective and potent k and d opioid antagonists, nor-binaltorphimine (4, nor-BNI) [7, 8] and naltrindole (5, NTI) [9–11], respectively (Fig. 2). This work was a landmark achievement in opioid research. Twin drugs are mainly utilized for two aims. One is the investigation of receptor dimerization; the other is the development of medicines with no or fewer adverse side effects. First, since the first evidence that k and d receptors must organize as heterodimers to be fully functional [12], numerous reports of hetero- and homodimerization of opioid receptors and other receptors have appeared [13–19]. Although opioid receptors have been historically classified into three types (m, d, and k types) and further divided into several subtypes from the pharmacological viewpoint [20], only the three major types have been cloned [21]. Receptor dimerization has been invoked to explain the discrepancy between widely varied pharmacologies and the identification of only three opioid receptor types. Twin drugs are expected to be useful tools for investigation of receptor dimerization and pharmacology derived from receptor dimers, because two pharmacophores in an appropriate twin drug would simultaneously bind to two identical or different binding sites in receptor dimers. Second, drug discovery has been based on the principle of “one molecule-one target-one disease,” but many diseases remained inadequately treated by such an approach and gradually it was recognized that simultaneous modulation of multiple targets would be more effective. The designed multiple ligand (DML) strategy is one of the methodologies to modulate multiple targets simultaneously [22–24]. As the DML has two pharmacophores interacting at respective targets, the DML and the twin drug represent the same thing. The DML is categorized into three types: conjugate, fused, and merged DML (Fig. 3). The difference among these DMLs is the distance between the two pharmacophores. The conjugate DML consisting of two pharmacophores linked to each other by a spacer has the longest distance between the two pharmacophores. The fused DML whose pharmacophores are linked with each other directly or via a short spacer possesses the middle distance between the two pharmacophores. The merged DML, sharing some part of each pharmacophore, has the shortest distance between the two pharmacophores. Many twin drugs belong to the conjugate or fused DML categories.


243

Fig. 3 DMLs consisting of two pharmacophores A and B. (a) Conjugate DML. (b) Fused DML. (c) Merged DML

Excellent reviews about twin drugs with opioid pharmacophores have been published [25–27], including those that discuss peptidic twin drugs [27–29]. The following chapter will mainly focus on the twin and triplet drugs whose in vivo pharmacological effects have been evaluated, and will include a description of their pharmacology and synthesis.

2 Twin Drugs Consisting of Two Opioid Pharmacophores Many twin drugs consisting of two opioid pharmacophores have been designed and synthesized for investigation of receptor dimerization and/or as DMLs. Some investigations using twin drugs have given insight into receptor subtype.

2.1

m and k Pharmacophores

Behavioral studies showed that k agonists with various activity at the m receptor more effectively reduced cocaine self-administration in nonhuman primates with fewer undesirable side effects than the highly selective k agonists [30–32], and thus symmetrical twin drugs possessing two cyclorphans (6) [33] or butorphans (7) [34], which are full k agonists and partial m agonists, were synthesized (Fig. 4) [35–38]. Compounds 8–11 bridged at the 2- or 17-position had low binding affinities for opioid receptors. These results are in agreement with reported structure-activity relationship: introduction of substituents at the 2-position lowers opioid activities [39]; 17-substituents influence the opioid activities and some bulky substituents decrease the affinities [39, 40]. Compounds 12 with an ether linker at the 3-hydroxy position of butorphan (7) also bound to opioid receptors with low affinities (Table 1). However, twin drugs 13 linked by ester functions at the 3-hydroxy position of cyclorphan (6) or butorphan (7) exhibited good binding affinities (Table 1). Twin drugs 14, which replaced the ester spacer with an amide one, more strongly bound to the opioid receptors than the compounds 12 which were linked by ether functionalities, but showed about 20- to 60-fold weaker binding affinities than twin drugs 13 with the ester linker. Thus, the affinity was very sensitive not only to the bridged position but also to the functionality of the linker. Very recently, twin drugs 15 and 16 with ether linkages containing hydroxy groups were reported to show high binding affinity (Table 1) [41]. This result again

244

H. Fujii

R 17 N

H

H N

(CH2)n

H N

3 2

OH

OH

HO 8

Cyclorphan (6): R = CPM Butorphan (7): R = CBM (cyclobutylmethyl) H

H N

N

OH HO N R

n

9

N R

H

H

N

N

OH HO HO

OH 10

n

H

H

N

N

OH HO O O N H

4

11

N H

H

H

N

N

O (CH2)n O 12

Fig. 4 Structures of cyclorphan (6), butorphan (7), and twin drugs 8–17


245

H

R

H

R

N

N

O

O

O

n O

R = CPM, CBM 13 MCL-139 (R = CBM, n = 2) (13a) MCL-144 (R = CBM, n = 8) (13b) H

H

N

N

O

O

N H

n N

H

14 H

H

N

N

O

O OH 15

H

H

N

N

O

6 OH OH

O

16 H N

O O

O O MCL-145 (17)

N H

Fig. 4 (Continued)

246

H. Fujii Table 1 Binding affinities of cyclorphan (6), butorphan (7), twin drugs 12, 13, and 15–17 to the opioid receptors Ki (d) (nM) Ki (k) (nM) Compound Ki (m) (nM) Cyclorphan (6) 0.062 1.9 0.034 Butorphan (7) 0.23 5.9 0.079 12a (n ¼ 3) 7.1 180 6.2 12b (n ¼ 10) 66 2,500 120 MCL-139 (13a) 0.16 9.4 0.076 MCL-144 (13b) 0.09 4.2 0.049 13c (R ¼ CPM, n ¼ 2) 0.14 3.8 0.10 13d (R ¼ CPM, n ¼ 8) 0.93 32 0.41 15 0.95 37 0.99 16 2.4 27 1.4 MCL-145 (17) 0.20 9.4 0.078

emphasizes the importance of linker functionality for binding ability. The phenolic 3-hydroxy group is also known to be one of the crucial functional groups for binding to the opioid receptors [39–41] and intensive searches for bioisosteres of the phenolic hydroxy group have been carried out [42–58]. Notably, some twin drugs substituted at the 3-hydroxy group displayed sufficient binding to the opioid receptors. Among the twin drugs with ester spacers, MCL-139 (13a, R ¼ CBM (cyclobutylmethyl), n ¼ 2), MCL-144 (13b, R ¼ CBM, n ¼ 8), and MCL-145 (17) strongly bound to the opioid receptors and their affinities were comparable to or higher than the parent compound 7 (Table 1). Twin drug MCL-144 derivatives having (+) butorphan were also synthesized (Fig. 5). (+)()-MCL-144 (13b0 ) had 30- to 108-fold stronger binding affinities than (+)(+)-MCL-144 (13b00 ) but 5- to 24-fold weaker binding than ()()-MCL144 (13b), agreeing with a previous report [59]. A good relationship between the affinities and the number of methylene units in the ester linker was observed and compound 13b with eight methylene units showed the highest affinity among analogs with two to ten methylene units except for compound 13a with an ethylene unit or 17 with a vinylene unit. In the functional assay ([35S]GTPgS binding assay), compounds 13a and 13b were partial m agonists and full k agonists (Table 2) [35]. On the other hand, compound 17 showed bell-shaped agonist activities at m and k receptors with a maximum [35S]GTPgS binding at around 107 M [60]. Intracerebroventricular (i.c.v.) administration of compound 17 produced dose-dependent antinociception (ED50 ¼ 0.27 nmol) in the mouse warm-water-tail-flick test, and its activities were more potent than butorphan (6) itself (ED50 ¼ 7.3 nmol) and equipotent to morphine. Antinociceptive effects induced by 17 were blocked by an i.c.v. treatment of m antagonist b-funaltrexamine (b-FNA) or k antagonist nor-BNI, but not by d antagonist NTI. Moreover, compound 17 at a low dose (0.05 nmol, i.c.v.) inhibited the morphine-induced antinociception [60]. In vivo pharmacological effects of the most potent agonist 13b were also evaluated. The twin drug 13b bridged with an ester linker with eight methylene units exhibited analgesic effects in a dose-dependent manner. Its potency (ED50 ¼ 3.0 nmol, i.c.v.) was about twice that of butorphan (7). Antinociception induced by i.c.v. administration of 13b was

Twin and Triplet Drugs in Opioid Research Fig. 5 Structures of twin drugs 13b–13b00

247 H

H

N

N

O O

O 8 O

(-)(-)-MCL-144 (13b) H

H

N

N

O O

O 8 O

(+)(-)-MCL-144 (13b') H

H

N

N

O O

O 8 O

(+)(+)-MCL-144 (13b'')

Table 2 Pharmacological properties of compounds MCL-139 (13a) and MCL-144 (13b) for m and k opioid receptors in [35S]GTPgS binding assay EC50 Imax IC50 Compound Receptor Pharmacological Emax (%) (nM) (%) (nM) type properties MCL-139 m Partial agonist 50 7.4 50 170 (13a) k Agonist 70 4.5 NIa NIa MCL-144 m Partial agonist 50 1.3 60 16 NIa (13b) k Agonist 60 0.85 NIa a No inhibition

antagonized by an i.c.v. treatment of b-FNA or nor-BNI, but not by d antagonist ICI 174,864 [61]. On the other hand, intraperitoneally (i.p.) administered compound 13b produced antinociceptive effects lasting no more than 2 h, but its analgesia was not blocked by i.p. treatment of nor-BNI. Moreover, pretreatment with 13b 150 min prior to administration of a k agonist U50,488 attenuated antinociception induced by the k agonist, suggesting that compound 13b would not be a k agonist, but rather a k antagonist when it was administered by the i.p. route [62]. These outcomes from i.c.v. or i.p. injection of 13b may result from a metabolite of 13b. It is worth noting that i.p. injection of compound 13b dose-dependently attenuated immobility in the forced-swim test, suggesting that the compound 13b may have antidepressant effects [61]. This observation is consistent with recent studies that activation of the k receptor would induce depression-like behaviors [63, 64] and vice versa [65–67].

248

H. Fujii

R

H

R

N

H

H

N

N

R

a O 6, 7

O

OH R = CPM, CBM X = -(CH2)n-,

O X

O

13, 17

Scheme 1 Synthesis of compounds 13 and 17. Reagents and conditions: (a) Et3N, ClCOXCOCl, CH2Cl2

There are no reports of the detailed pharmacological effects of compound 13a. In general, ester moieties are labile to chemical and enzymatic hydrolysis. Therefore, it is not clear that the observed pharmacological effects of these twin drugs with ester spacers were derived from the twin drugs themselves or from their metabolites. Since the half-life of compound 13b was 46 min in a phosphate buffer (pH 7.4) at 37 C1 [37], it was unlikely that metabolites of the twin drugs, at least compound 13b, were mainly responsible for in their in vitro pharmacological effects. Although the half-life of 13b was 70 min in the rat brain homogenate [61], further investigation would be needed to understand its in vitro pharmacological effects. Twin drugs 13 and 17 with ester spacers were synthesized by coupling reaction of compounds 6 or 7 with the appropriate diacyl chloride (Scheme 1). Twin drug MCL 450 (19) consisting of butorphan (7) and the d selective peptidic antagonist Dmt-Tic (18) [68] has also been reported (Fig. 6) [69]. MCL 450 (19) was synthesized by coupling reaction of butorphan (7) with Boc-b-Ala-OH and subsequent deprotection followed by coupling reaction with protected Dmt-Tic using EDC and HOBt (Scheme 2). MCL 450 (19) inherited the in vitro pharmacological features of each pharmacophore 7 and 18 to bind strongly with all three opioid receptors (Table 3) and to be a m and k agonist and a d antagonist (Table 4). Twin drugs 25–27 including butorphan (7) and the other pharmacophore nalbuphine (22: m agonist/antagonist, k agonist,), naltrexone (23: m antagonist, k antagonist), or naloxone (24: m antagonist, k agonist/antagonist) were also synthesized (Fig. 7) and their in vitro pharmacological profiles were evaluated (Tables 5–7). However, their in vivo pharmacological effects were not reported [70]. Very recently, the KMN series of twin drugs 30 tethering the m antagonist b-naltrexamine (28) [71] pharmacophore to the selective k antagonist 50 -guanidinonaltrindole (50 -GNTI, 29) [72, 73] pharmacophore were designed for investigation of k–m heterodimerization (Fig. 8) [74]. While KMN twin drugs 30 with spacers of various length, including KMN-21 (30a, n ¼ 3) itself, inhibited DAMGO (m agonist) and U69,593 (k agonist) induced Ca2+ release in preparations 1 It is generally accepted that the rate of a reaction doubles with increase in temperature of 10 C. Therefore, the rate of hydrolysis of the twin drugs at 25 C (incubating temperature applied in in vitro assays) is predicted to be approximately one-half of that at 37 C.


249 HO

H N N

H2N O

OH Butorphan (7)

O OH Dmt-Tic (18)

HO H N N

H2N O

O

O N O H MCL 450 (19)

Fig. 6 Structures of Butorphan (7), Dmt-Tic (18), and MCL 450 (19) H

H

N

N a

b, c O N H

O

OH 7

Boc

20

HO H N

HN Boc O

d

N O

O N H

MCL 450 (19)

O 21

Scheme 2 Synthesis of MCL 450 (19). Reagents and conditions: (a) Boc-b-Ala-OH, EDC, DMAP, CH2Cl2, rt, overnight, 50%; (b) TFA, CH2Cl2, rt, 2 h, 90%; (c) Boc-Dmt-Tic, EDC, HOBt, NMM, DMF, 0 C to rt, 27 h, 83%; (d) TFA, CH2Cl2, rt, 2 h, 92%

Table 3 Binding affinities of butorphan (7), Dmt-Tic (18), and MCL 450 (19) to the opioid receptors Compound Ki (m) (nM) Ki (d) (nM) Ki (k) (nM) Butorphan (7) 0.23 5.9 0.079 Dim-Tic (18) 894 1.6 37,500 MCL 450 (19) 0.69 1.5 0.28

250

H. Fujii

Table 4 Pharmacological properties of MCL 450 (19) for the m, d, and k opioid receptors in [35S] GTPgS binding assay Receptor type Pharmacological properties Emax (%) EC50 (nM) Imax (%) IC50 (nM) m Agonist/antagonist 79 8.9 47 97 d Antagonist 18 2.8 84 3.9 k Agonist 130 6.0 NIa NIa a No inhibition OH

OH

OH N

N

N O

OH

O

3

O

OH

3 OH

nalbuphine (22)

naltrexone (23)

O

O

3 OH naloxone (24)

H N

O O

O 8 OR

25: R = 22-3-yl 26: R = 23-3-yl 27: R = 24-3-yl

Fig. 7 Structures of nalbuphine (22), naltrexone (23), naloxone (24), and twin drugs 25–27 Table 5 Binding affinities of compounds 22–27 to the opioid receptors Ki (d) (nM) Compound Ki (m) (nM) Nalbuphine (22) 0.89 240 Naltrexone (23) 0.23 38 Naloxone (24) 0.79 76 25 0.46 28 26 0.29 15 27 0.43 39

Ki (k) (nM) 2.2 0.25 1.1 0.34 0.12 0.13

Table 6 Pharmacological properties of compound 22–27 for the m opioid receptor in [35S]GTPgS binding assay Compound Pharmacological properties Emax (%) EC50 (nM) Imax (%) IC50 (nM) Nalbuphine (22) Agonist/antagonist 47 14 74 110 79 17 Naltrexone (23) Antagonist 6.7 NAa 92 23 Naloxone (24) Antagonist 13 NAa 25 Antagonist 5.1 NAa 94 18 26 Agonist/antagonist 43 4.4 50 160 27 Agonist/antagonist 34 2.0 73 25 a Not applicable


251

Table 7 Pharmacological properties of compounds 22–27 for the k opioid receptor in [35S]GTPgS binding assay Compound Pharmacological properties Emax (%) EC50 (nM) Imax (%) IC50 (nM) Nalbuphine (22) Agonist 81 27 NIa NIa b Naltrexone (23) Antagonist 10 NA 67 200 Naloxone (24) Agonist/antagonist 25 10 55 320 25 Agonist/antagonist 67 3.2 39 600 26 Agonist 76 3.4 NIa NIa a 27 Agonist 86 1.6 NI NIa a No inhibition b Not applicable

OH

H N

OH

N

N O

NH2

NH

N O H

OH

NH2

OH

β-naltrexamine (28)

5'-GNTI (29) OH H N

OH N N OH OH

H N NH

O

H N n

O

N H

H N O

2

O

N

N O 2H HO

30 (n = 2-5) KMN-21 (n = 3) (30a)

Fig. 8 Structures of b-naltrexamine (28), 50 -GNTI (29), and KMN twin drugs 30

of cells singly expressing m and k receptors, only KMN-21 (30a) antagonized both DAMGO and U69,593 induced Ca2+ release in the cells that expressed both the m and k receptors together (m and k receptors coexpressing cells). These results suggest that KMN-21 (30a) with a linker containing 21 atoms would selectively antagonize the activation of k–m heterodimers. Compounds 30 were synthesized by coupling compound 32, having the oligoglycine unit, with N-aminoalkyl 50 -GNTI 35 in a reaction mediated by DCC and HOBt, which was prepared by guanidylation of 50 -amino-NTI 36 [75] with thiourea 34 in the presence of HgCl2 [76, 77] (Scheme 3).

2.2

d and k Pharmacophores

To investigate the d – k heterodimerization in the spinal cord [78–80], the KDN series of twin drugs 37 incorporating the selective d antagonist NTI (5) [9–11] and

252

H. Fujii OH O

N O

28

+

NH2

HO

O

H N 2

N H

O

O O

31

OH

a, b

t-Bu

2

OH O

N N O H

2

OH

H2N

n NHBoc

O

H N

N H

O

Cbz

33

O

2

32

N H

34

H2N n H N NH

OH

S

c

OH

d, e N H

OH

n NHBoc

NH2

N N O H

N

NCbz

N O H OH

35 f, g 30

OH 36

Scheme 3 Synthesis of compounds 30. Reagents and conditions: (a) DCC, HOBt, DMF; (b) TFA, CH2Cl2; (c) Cbz-Cl, KSCN; (d) 36, HgCl2, Et3N; (e) HCl/Et2O; (f) 32, DCC, HOBt, DMF; (g) cyclohexadiene, Pd/C

the selective k antagonist 50 -GNTI (29) [72, 73] were designed (Fig. 9) [81]. The spacer consisted of glycine units and the alkyldiamine moiety. The former maintains a favorable balance between hydrophilicity and lipophilicity, and the latter adjusts the length of the spacer by one atom unit. Twin drugs 37 were synthesized in the manner similar to the synthesis of compounds 30 (Scheme 4). Among the synthesized twin drugs, KDN-21 (37a, n ¼ 2, m ¼ 3) showed 200-fold stronger binding affinity in the d and k receptors coexpressing cells than in the mixed preparation of cells expressing the d and k receptors singly. Moreover, NTI and 50 -GNTI derivatives 38 and 39 having a spacer whose length corresponds to that of 37a exhibited very weak binding affinities in the d and k receptors coexpressing cells (Table 8). These outcomes suggested that the influence of the spacer had negligible effects on binding and that KDN-21 (37a) would bridge the d and k binding sites, that is, the d – k heterodimer. In the mouse tail flick test, intrathecal (i.t.) administration of 37a effectively attenuated the antinociceptive effects induced by i.t. injection of DPDPE (d1 agonist) [82] or bremazocine (k2 agonist)


253

OH

H N

OH

N

N N O H

NH

N O H

OH

NH2

OH

NTI (5)

5'-GNTI (29) HO

OH N N OH HN

O O

OH

H N

N H

N nH

O

HO N

NH

O

N

N H

mN

H

OH

n

37 (n = 1, 2; m = 2-5) KDN-21 (n = 2, m = 3) (37a) OH N N OH OH

O HN

H N

N H

O

O

O N

2H

O

3N

H

2

38 H N

OH N

H N NH

N OH

O

H N 3

O

N H 2

H N O

O N

2H

OH 39

Fig. 9 Structures of NTI (5), 50 -GNTI (29), KDN twin drugs 37, and compounds 38 and 39

[83, 84], but was almost ineffective against deltorphin II (d2 agonist) [85] or U50,488 (k1 agonist) [83, 86]. In contrast, i.c.v. administered 37a produced analgesic effects (ED50 ¼ 31 pmol) but the low dose of 37a, which showed no analgesia, exhibited antagonism whose profile was different from that obtained by the i.t. route: DPDPE induced antinociception was antagonized but analgesia produced by bremazocine, U50,488, or deltorphin II was not. Taken together, this outcome suggested that KDN-21 (37a) bound the d – k heterodimer in the spinal cord but not in the brain and that the d – k heterodimer seemed to correspond to the d1 and k2 subtypes. The receptor subtype selectivity of 37a was also confirmed by mitogen-activated protein kinase (MAPK) activity assay [87]. Tethering the selective d antagonist NTI (5) [9–11] with the k1 agonist ICI 199,441 (43) [88] gave the KDAN series of twin drugs 44 (Fig. 10) [89]. Twin drugs 44 were synthesized in a similar manner to the synthesis of compound 30. In the

254

H. Fujii OH N

O N O H

+

HO

O

H N n

NH2

O 41

OH

O

N H

O

a, b

R

n

R = Bn, t-Bu

40 OH N O N O H

O

H N

HN

n

N H

O

OH O

OH

c, d

37

n

42

Scheme 4 Synthesis of compounds 37. Reagents and conditions: (a) DCC, HOBt; (b) 1 M HCl, AcOH, or H2 (80 psi), Pd/C; (c) 35, DCC, HOBt; (d) H2 (85 psi), Pd/C

Table 8 Binding affinities of KDN-21 (37a) and compounds 38 and 39 to the d and k receptors coexpressing cell or the mixed cells expressing the d and k receptors singly Compound Ki (coexpressed d k) (nM) Ki (mixed d þ k) (nM) KDN-21 (37a) 0.3 63 38 >1,000 199 39 >1,000 54.8

OH

Me N

N N N O H

Cl

O

Cl

OH NTI (5)

ICI 199,441 (43) N

OH

Me N O

N

O N O H OH

N H

H N O m

H N n O

O N H m

44 (m = 1, 2; n = 2, 4) KDAN-18 (m = n = 2) (44a)

Fig. 10 Structures of NTI (5), ICI 199,441 (43), and KDAN twin drugs 44

Cl Cl


255

Table 9 Binding affinitiesa of KDAN-18 (44a) to the d and k receptors coexpressing cell or the preparation of mixed cells separately expressing the d and k receptors singly in the presence or absence of d antagonists Ki (mixed d þ k) (nM) Antagonist Ki (coexpressed d – k) (nM) None 0.06 2.51 BNTX 0.19 1.14 NTB 2.34 1.94 a 3 [ H] U69,593 was used

binding assay using [3H] deltorphin II (d2 agonist) or [3H] U69,593 (k1 agonist) [83], KDAN-18 (44a, m ¼ n ¼ 2) showed respective 65- or 42-fold higher affinities in the d and k receptors coexpressing cell as compared to the mixed cells singly expressing the d and k receptors ([3H] deltorphin II: Ki (coexpressed d – k) ¼ 0.008 nM, Ki (mixed d þ k) ¼ 0.52 nM; [3H] U69,593: Ki (coexpressed d – k) ¼ 0.06 nM, Ki (mixed d þ k) ¼ 2.51 nM). The binding affinity of 44a, as evaluated by [3H] U69,593 in the d and k receptors coexpressing cells, significantly decreased in the presence of NTB (d2 antagonist) [90, 91], but not with BNTX (d1 antagonist) [92, 93] (Table 9). Moreover, the antinociceptive effects induced by i.t. administered 44a were attenuated by i.t. treatment of NTB or nor-BNI (k1 antagonist) [94] but not by i.t. injection of BNTX. Based on these results, it was proposed that KDAN-18 (44a) would bridge neighboring d and k homodimer, giving rise to d2 and k1 subtypes. This proposal was supported by the observation that KDN-21 (37a), which bound to the d – k heterodimer that would give rise to d1 and k2 subtypes, did not affect the antinociception induced by KDAN-18 (44a) [89].

2.3

m and d Pharmacophores

The m and d receptors interact with each other to potentiate morphine-induced antinociception [95] and to reduce morphine tolerance and physical dependence [96–100]. To investigate the participation of direct association between the m and d receptors in the pharmacological phenomena mentioned above, the MDAN series of twin drugs including the m agonist oxymorphone (45) [71] and the selective d antagonist NTI (5) [9–11] pharmacophores were designed (Fig. 11) [101]. These twin drugs were synthesized by a process similar to the synthesis of compound 30. In the mouse tail-flick test, i.c.v. administration of MDAN-19 (46a, n ¼ 5) and MDAN-21 (46b, n ¼ 7) produced antinociception and their antinociceptive effects (46a: ED50 ¼ 0.43 nmol; 46b: ED50 ¼ 0.08 nmol) were less potent than oxymorphone (ED50 ¼ 0.043 nmol), but more potent than morphine (ED50 ¼ 4.1 nmol). Notably, the twin drugs 46a and 46b showed analgesic effects by not only i.c.v., but also intravenous (i.v.) administration [102]. Although i.c.v. pretreatment with d antagonist NTI profoundly increased the

256

H. Fujii

Me

OH

OH N

N O

N O H

O

OH oxymorphone (45) Me

OH NTI (5) OH

OH N

O N OH

O O

O N H

nN

N

O O

H

N H

N HO HO

OH 46 (n = 2-7) MDAN-19 (n=5) (46a) MDAN-21 (n=7) (46b)

Fig. 11 Structures of oxymorphone (45), NTI (5), and MDAN twin drugs 46

acute analgesic potency induced by i.c.v. injection of 46a or other twin drugs having a shorter spacer length, the effect of i.c.v. pretreatment with NTI on the acute antinociceptive potency produced by i.c.v. administration of 46b was marginal, suggesting that 46b would more effectively interact with the m–d heterodimer.2 Chronic effects were also evaluated using mice to which test compounds were i.c.v. infused for 3 days (chronic treatment). The potency of morphine-induced antinociception revealed a sixfold decrease (tolerance) and subcutaneous (s.c.) treatment with the opioid universal antagonist naloxone induced jumping (a sign of drug dependence) in the mice chronically treated with morphine. On the other hand, the antinociceptive potency of the twin drugs 46a and 46b did not change and almost no naloxone-induced jumping was observed in the mice chronically treated with 46a or 46b, respectively. Moreover, compounds 46a and 46b exhibited neither place preference nor place aversion in the conditioned place preference (CPP) test [77]. These results strongly suggest that the twin drugs 46a and 46b would be novel and potent analgesics without side effects of tolerance and dependence. In the patent literature [103], i.v. administration of twin drug 47 consisting of the m antagonist b-naltrexamine (28) [71] and the selective d antagonist NTI (5) [9–11] pharmacophores was reported to reduce plasma glucose levels. Unfortunately, there have been no descriptions of the in vitro pharmacological profiles (Fig. 12).

2

The authors proposed that MDAN-21 (46b) having a longer spacer could bridge the m and d receptors in multiple modes and that binding by MDAN-19 (46a) with a shorter linker would be less efficient than that by MDAN-21 (46b). As a result, the acute analgesic potency of 46a would be affected by treatment with NTI.


257

OH

OH N

N O

N O H

NH2

OH β-naltrexamine (28)

OH NTI (5)

OH N

HO N O H

H O N

HN S

OH

N OH

47

Fig. 12 Structures of b-naltrexamine (28), NTI (5), and twin drug 47 HO

O

O

O

O N

N

N O OH OH NS 28 (48)

O

O

O OH NS 29 (49)

Fig. 13 Structures of NS 28 (48) and NS 29 (49)

2.4

Symmetrical Twin Drug

Although most of recently reported twin drugs are nonsymmetrical, symmetrical versions have also been reported. Twin drug NS 29 (49) (Fig. 13) [104] consisting of two NS 28 (48) units (m antagonist) [105] with a 1,4-dioxane spacer was synthesized as shown in Scheme 5. The key reaction was conversion of the inseparable mixture of compounds 54–56 into compound 57 under azeotropic reflux conditions. The structure of 57 was determined by X-ray crystallography. NS 28 (48) blocked only the morphine-induced [35S]GTPgS binding, while twin drug NS 29 (49) attenuated the [35S]GTPgS binding induced by morphine (m agonist), U50,488 (k agonist), and b-endorphin (putative e agonist [106]), but not by SNC-80 (d agonist), suggesting that dimerization of ligands could affect the selectivity of the monomer pharmacophores for different types of receptors.

258

H. Fujii OH

OMe

N OH

OH

N

N O

O OH

a

S

O OH

O

OMe

S b

52

OMe

+ 50

OMe

51

OMe

O

OMe N

O OH OH N

53

CHO O OH

OH N

OMe

c

54 O

MeO OH OO OO HO

OH

N OH

OMe 56

N

OMe

O OH

55

OMe MeO

O

O d

O N

N O

e

O

NS 29 (49)

O OMe 57

Scheme 5 Synthesis of NS 29 (49). Reagents and conditions: (a) 1,3-dithian, n-BuLi, THF, 70 C to 60 40 C, 85%; (b) CH(OMe)3, CuCl2/CSA, MeOH, 50 C, 52: 65%, 53: 18%; (c) 1 M HCl, reflux; (d) CSA, toluene, reflux, 23%; (e) t-BuOK, n-PrSH, DMF, 150 C, 82%

3 Twin Drugs Consisting of Opioid and the Other Pharmacophores Twin drugs consisting of opioid and the other pharmacophores have also been designed. Although many of the twin drugs belonging to this category are peptidic [107–119], non-peptidic twin drugs and non-peptide–peptide hybrids will be featured in this section.


3.1

259

Opioid and Imidazoline Binding Site Pharmacophores

The imidazoline binding site (IBS) was discovered as an imidazoline preferring binding site of the well-known antihypertensive drug clonidine in 1984 and classified into two types (I1 and I2) [120]. IBS ligands were reported to modulate antinociception [121, 122], tolerance [123–125], and dependence [126–128] induced by opioids. In a preliminary study [129], fentanyl (58) and the guanidine moiety, which mimics an endogenous I2–IBS ligand agmatine (59) [130], were chosen as the opioid and I2–IBS pharmacophores, respectively, to provide twin drugs 61 and 62. Compounds 61 and 62 decreased the binding affinities for IBS, while 61b and 62b, having an acyclic guanidine moiety, maintained the affinities for opioid receptors (Fig. 14, Table 10). Compounds 61b and 62b showed opioid agonistic activities in the guinea pig ileum (GPI) assay, but their respective potencies were 9- and 31-fold weaker than that of morphine. I.p. administration of compounds 61b or 62b exhibited dose-dependent and naloxone-reversible antinociception in the mouse acetic acid writhing assay (61b: 50.7% inhibition of writhes at 20 mg/kg, 62b: 47.4% inhibition of writhes at 25 mg/kg). Although the O N

H N

H2N

N

NH2 NH

fantanyl (58)

N

N HN

agmatine (59)

BU224 (60)

O

O N

R

N

R

N

N

61a, b

62a, b NH

a: R =

N

NH

b: R =

NH2

HN

HN N

O N N

N

O H N n

NH2 NH

63a-h (n = 2-9, 12)

N N

H N n

O

64a-d (n = 3, 6, 8, 12)

Fig. 14 Structures of fentanyl (58), agmatine (59), BU224 (60), and twin drugs 61–64

N H

260

H. Fujii

Table 10 Binding affinities of compounds 61 and 62, idazoxan, and fentanyl (58) to the opioid and I2-imidazoline receptors Compound Ki (m) (nM) Ki (d) (nM) Ki (k) (nM) Ki (I2) (nM) Idazoxana NDb NDb NDb 307 6 663 298 5,462 Fentanyl (58)c 61a 7,119 12,078 11,551 9,630 61b 7.8 181 841 1,890 62a 1,751 24,955 5,043 2,327 62b 37 1,744 355 2,022 a Idazoxan was used as a control compound for the I2 ligand b Not determined c Fentanyl was used a control compound for the opioid ligand

Table 11 Binding affinities of compounds 63 and 64, idazoxan, and fentanyl (58) to the m opioid and I2-imidazoline receptors Compound n Ki (m) (nM) Ki (I2) (nM) – NDb 28 Idazoxana Fentanyl (58)c – 2.9 8,593 63a 2 433 437 63b (¼62b) 3 23 1,920 63c 4 0.59 >10,000 63d 6 1.04 409 63e 7 0.37 6,627 63f 8 37 126 63g 9 26 58 63h 12 477 6.5 64a 3 6,142 875 64b 6 2,168 323 64c 8 339 >10,000 64d 12 545 547 a Idazoxan was used as a control compound for the I2 ligand b Not determined c Fentanyl was used a control compound for the m opioid ligand

outcome seems to indicate that compound 61b was superior to compound 62b as a lead compound for an analgesic, further investigation of compound 62 derivatives was carried out [131, 132]. Twin drugs 63 having various chain lengths were synthesized and evaluated for their binding affinities (Fig. 14, Table 11). The optimal chain length was four to seven carbon units for the m opioid receptor, while the best binding affinity for I2-IBS was observed in compound 63h with the longest spacer. Although the twin drugs 64 incorporating the BU224 (60, I2-IBS ligand) structure [133] were also prepared (Fig. 14), their binding affinities for both receptors were comparable to or weaker than those of compounds 63. Compound 63d, which showed the highest m opioid receptor along with an acceptable I2-IBS affinity, induced [35S]GTPgS binding and its [35S]GTPgS binding was attenuated by naloxone. However, the maximum binding (Emax) of 63d was about half the value of that of DAMGO (m full agonist), indicating that 63d would have a partial

Twin and Triplet Drugs in Opioid Research H2N (CH2)n NHBoc 65a-h + O

261 H N

a

H N n

N

Boc

N 67a-h 66 O N b

N

H N n

Boc

c, d, c

63a-h

68a-h

Scheme 6 Synthesis of twin drugs 63. Reagents and conditions: (a) NaBH3CN, MS 3A, MeOH, rt, 50–82%; (b) (EtCO)2O, DMAP, py, CH2Cl2, rt, 53–86%; (c) TFA, CH2Cl2, 60–99%; (d) BocNH (C=S)NHBoc, HgCl2, Et3N, CH2Cl2, rt, 50–95%

m agonist activity. Unfortunately, 63d produced no antinociception in both the acetic acid writhing and the hot-plate tests. These outcomes may stem from low efficacy for the m opioid receptor, poor permeability of the blood-brain barrier, or both. Synthesis of twin drugs 63 commenced with reductive amination of Nphenethylpiperidone (66) with mono-Boc-protected alkyldiamines 65 (Scheme 6). Introduction of the guanidine moiety was completed by the reaction of the amine derived from Boc-protected amine 68 with Boc-protected thiourea in the presence of HgCl2 [76, 77].

3.2

Opioid and Substance P Pharmacophores

The neuropeptide substance P (SP) and endogenous opioid peptides are intimately involved in the regulation and modulation of pain transmission [134–139]. SP is known to enhance analgesia and to inhibit tolerance induced by opioids [108, 140, 141]. Based on such background information, several peptidic twin drugs with opioid and SP pharmacophores have been reported [108–112]. Recently a chimeric twin drug MPS9 (69) containing morphine and N-acetyl SP3–11 ((a-NHAc)LysPro-Gln-Gln-Phe-Phe-Gly-Leu-Met-NH2) pharmacophores with a succinate linker have been synthesized (Fig. 15) and showed moderate to strong binding affinities (approximate affinity: 107 M (m and k receptors), 106 M (d receptor), 109 M (SP receptor)) [142]. Intramuscular (i.m.) administration of compound 69 elicited bell-shaped antinociceptive effects with maximum efficacy at 0.2 mg/kg. This phenomenon was also observed when peptidic twin drugs [107, 108] or coadministration of both morphine and SP [140, 141] were applied. The analgesic effects

262

Me

H. Fujii H N

O O

O

H N

O

Pro Gln Gln Phe Phe Gly Leu Met NH2 NHAc

O

OH

MPS9 (69)

Fig. 15 Structure of MPS9 (69)

produced by 69 were attenuated by L733060 (SP antagonist) or nor-BNI (k antagonist). Interestingly, twin drug 69 exhibited antinociception in morphine-tolerant rats. Compound 69 was prepared by reaction of morphine with succinic anhydride to give morphine 6-hemisuccinate [143] and subsequent coupling reaction with the SP3–11 fragment using N-hydroxysuccinimide.

3.3

Opioid and Cholecystokinin Pharmacophores

Cholecystokinin (CCK) is known to have anti-opioid effects [144–146] and CCK antagonists potentiate opioid-induced antinociception and prevent tolerance and dependence by opioids [147–149]. Based on this information, several peptidic twin drugs consisting of an opioid and CCK pharmacophores have been reported [113– 118]. Recently, to investigate the possible existence of heterodimeric m opioid/ CCK2 receptors, which may be responsible for the pharmacological observations mentioned above, the twin drugs 71 and 72 possessing the m agonist oxymorphone (45) [71] and the CCK2 antagonist L-365,260 (70) [150] with spacers of various lengths were designed (Fig. 16) [151]. From the bioluminescence resonance energy transfer (BRET) studies [152], it is suggested that the m opioid receptor and the CCK2 receptor would form homodimers in the COS cells. Whereas compound 71a, possessing an 18 atom unit spacer, increased the BRET ratio in COS cells expressing both tagged m opioid and CCK2 receptor constructs, compound 72, having a shorter spacer, did not influence the BRET ratio. The increment of the BRET ratio induced by 71a was significantly attenuated by the CCK2 antagonist 73 with a linker. These results suggest that compound 71a can simultaneously bind to both receptors but compound 72 cannot. On the basis of the results obtained, it was proposed that the m opioid and CCK2 receptors coexpressed in COS cell would be present as more stable homodimers composed of the individual receptors and that an appropriate twin drug which can spontaneously bind to each type of receptor would induce heterodimerization of these receptors. Moreover, in a tolerance study using the mouse tail-flick test, m agonist 74 with a spacer produced tolerance but twin drugs 71a and 72 induced no significant tolerance. Taken together, these results suggest that the heterodimerization of m opioid and CCK2 receptors would not be relevant to blockade of tolerance. The twin


Me

OH

263

H N

Me

N

H N

N

O O

O

OH oxymorphone (45)

Me

Me N

O

L-365,260 (70)

OH N

O

O O

N OH

O N H

n

N H

OH

Me

O O

H N

O

N H

H N

O

Me N

N

O

71 (n = 0, 2, 6) 71a (n = 2) OH

N

O

O O

N OH

H N

O

N H

H N

Me N

O

O

N

H N

H N

OH 72

O N H

O O

O N H

6

O O

N H

O

N H

O

Me N

N

O

73

Me

OH N

O N OH

O O

O N H

4

N H

O O

N H

OH 74

Fig. 16 Structures of oxymorphone (45), L-365,260 (70), twin drugs 71 and 72, and compounds 73 and 74

drugs 71a and 72 were synthesized as shown in Scheme 7. Chiral ()-75 obtained by optical resolution [153] of racemic 75 [154] was converted into isocyanate 76 by treatment with triphosgene. Oxymorphamine derivatives 80 and 81 having the corresponding spacer were prepared by a procedure similar to the synthesis of 30 followed by reduction of the nitro group in precursors of 80 and 81 using Pd/C catalyst. The reaction of the amino group in the resulting compounds 80 and 81 with isocyanate 76 provided twin drugs 71 and 72.

264

H. Fujii Me N

Me N

O NH2

O NCO

a

N

N

76

(-)-75 Me

OH

OH

Me

N O

N

O

b

NH2

N O H

OH

78

OH N

O

O O

N O H OH Me

c, d

OH

OH

77 Me

O O

N H

79

OH N

O

O

O

O

N O H

N H

OH g

O O

nN

H

N H

O

NH2

80

71

Me 70

e, f

nNH2

b, h, f

OH N

O N O H

O O

N H

O

NH2

g

72

OH 81

O HO

O O

N H 82

O

NO2

H2N

O

NO2

83

Scheme 7 Synthesis of twin drugs 71 and 72. Reagents and conditions: (a) CO(OCCl3)2, sat. Na2CO3aq/CH2Cl2, 0 C; (b) diglycolic anhydride, CH2Cl2, rt; (c) mono-Boc-diamines, DCC, HOBt, DMF; (d) TFA, CH2Cl2; (e) 82, DCC, HOBt; (f) H2 (50 psi), Pd/C, MeOH; (g) 76, DMF; (h) 83, DCC, HOBt, DMF


3.4

265

Miscellaneous

Twin drug 85 consisting of fentanyl (58) (m agonist) and indomethacin (84) (NSAID) pharmacophores [155], twin drugs 89 and 90 containing tramadol (86) (m agonist) and metoclopramide (87) (5-HT4 agonist) or cisapride (88) (5-HT4 agonist) pharmacophores [156], and twin drug 91 including fentanyl (58) (m agonist) and adenosine (92) (A1 agonist) pharmacophores [157] were synthesized (Fig. 17). Only the in vitro pharmacological properties have been reported for MeO MeO

O

CO2H

CO2H N

N Me

N O

N

Me

N

Cl fentanyl (58)

85

indomethacin (84) F

MeO

OH

Et2N

tramadol (86)

Cl

N H MeO

NMe2

NH

metoclopramide (87)

cisapride (88)

OMe

N

cis

H N

N Me

NH O

Cl

O

OMe R: H, Me 90

R: H, Me 89

NH2

MeO

O

NH2

HN N H N

N

N O

N O

N N

N

N

HO O OH

91

NH2

MeO

OH

RO NH2

OH

Cl

O

NH2

Cl RO

OMe

N

O

O

OH

Fig. 17 Structures of compounds 58 and 84–92

OH

OH

adenosine (92)

N N

266

H. Fujii

compounds 85, 89, and 90, whereas compound 91 was shown to attenuate the antinociception induced by DAMGO (m agonist), CPA (A1 agonist), or coadministration of DAMGO and CPA in the rat tail-flick and hot-plate tests (in vivo tests).

4 Triplet Drugs Triplet drugs are defined as compounds that contain three pharmacophores in a single molecule. As mentioned earlier, although twin drugs can only play one role, either by an increase of activity (symmetrical twin drug) or by a dual action (nonsymmetrical twin drug), triplet drugs are expected to elicit both roles. That is, nonsymmetrical triplet drugs with two of the same pharmacophores and one different moiety may exhibit both increased pharmacological action and dual action because the two identical portions could bind the same receptor sites simultaneously while the third portion could bind a different receptor site or enzyme. Although triplet drugs are predicted to be useful pharmacological tools and to serve as lead compounds for developing novel drugs, there have been no reports of the triplet drugs except for natural products such as valinomycin (93) [158, 159] and enterobactin (94) [160] (Fig. 18). Recently, novel triplet drugs having 1,3,5trioxazatriquinane cores were synthesized [161]. S.c. administration of symmetrical triplet drug 95 having three oxymorphone (m agonist) units exhibited profound antinociceptive effects in the acetic acid writhing test (ED50 ¼ 0.037 mg/kg) that were 16- and 4-fold more potent than that of morphine (ED50 ¼ 0.6 mg/kg) and symmetrical twin drug 96 with a capped structure (ED50 ¼ 0.14 mg/kg), respectively. Given that their ED50 values are converted from gram units to molar units, the potency of triplet drug 95 for its antinociception (ED50 ¼ 0.039 mmol/kg) was 54- and 5-fold higher than that of morphine (ED50 ¼ 2.1 mmol/kg) and symmetrical twin drug 96 (ED50 ¼ 0.20 mmol/kg), respectively (Fig. 19). Compounds 95 and its N-CPM derivative 99 were synthesized as shown in Scheme 8. Isolation of the intermediates 97 is significant for synthesis of symmetrical twin drugs with a capped structure like 96 or nonsymmetrical triplet drugs 102 (Scheme 9). According to this method, nonsymmetrical triplet drug 103 possessing two opioid pharmacophores and one haloperidol (antipsychotic) moiety has been prepared. However, its pharmacological effects were not reported (Fig. 20). Attempts to prepare triplet drugs 104 having morphinan structures without a 4,5-epoxy bridge gave not the desired compound 104 but the unexpected rearrangement product 105 [162] (Fig. 21).

5 Conclusion and Perspectives Many twin drugs and triplet drugs have been synthesized and evaluated for their pharmacological effects including in vivo pharmacological effects. Some compounds, such as MCL-144 (13b), MCL-145 (17), and triplet drug 95, showed


267

more potent antinociceptive effects than the corresponding monomeric agonists. Some compounds were designed with the purpose of reducing morphine-induced side effects such as tolerance and dependence. When MDAN-19 (46a) and MDAN-21 (46b) with m agonist and d antagonist pharmacophores were chronically administered, naloxone-induced dependence signs (jumping) were not observed. Compounds 46a and 46b also exhibited neither place preference nor place aversion in CPP tests. MPS9 (69) with m agonist and SP pharmacophores produced antinociceptive effects in morphine tolerant rats. These outcomes would give impact to development of novel analgesics with few or no side effects. However, accumulation of further investigations of pharmacological properties, especially in vivo CH3 O H3C

H3C

CH3 H C C O O C N H HC H3C CH L D C NH O L H3C O CH3 HC C O D CH C H3C D HN O CH3 HC H3C O C L CH D C O H3C

NH

O

H3C H3C

O

L

O

CH3 HC C CH L C O CH3 D O HN HC D C L CH3 CH C H CH3 O O C C N H O H3C H C 3 O CH3 H3C

valinomycin (93) HO HO HN

O

O O

O N H OH

O

O

O O

NH O

OH enterobactin (94)

Fig. 18 Structures of valinomycin (93) and enterobactin (94)

OH OH

268

H. Fujii Me N OH

HO

OO

O O

OH

HO

OO O

N N O O

HO

OH

N N

Me

O O

HO

OH

OH Me

Me

OH

N

N

Me 95

96

Fig. 19 Structures of triplet drug 95 and twin drug 96 with a capped structure OH N

O

N

CHO

O

OMe

OMe

MeO OH O O

N

OH

O OH

54

OH

OH

O O

N

OH OMe

55

OH

56 MeO

OH a

N

N HOO OO

b

*

N OH

OMe (R) - and (S)-97

N

N OH MeO

OMe HO

O O

OH

OO c

N N O O

OH

OH HO

O O

OO N N O O

OMe N

HO

OH OH

N

98

99

Scheme 8 Synthesis of triplet drug 99. Reagents and conditions: (a) NH4Cl, AcONa, MeOH, reflux, (R)-97: 46%, (S)-97: 9%; (b) CSA, CHCl3, reflux, 68% from (R)-97; (c) BBr3, CH2Cl2, rt, 80%


269 RO R

R' OO

O (R)- and (S)-97 +

HO CHO R'

R

N

a

N O O

HO

100

OH OR

N

b

101: R = Me 102: R = H

Scheme 9 Synthesis of twin drug 102. Reagents and conditions: (a) CSA, CHCl3, rt; (b) BBr3, CH2Cl2, rt

Fig. 20 Structure of nonsymmetrical triplet drug 103

Cl

OH N F

MeO OO O

N N

HO

O O

OH OMe

N 103

pharmacological ones, as well as absorption, distribution, metabolism, and excretion (ADME), toxicological, and physicochemical evaluation would be required in order to validate the possibilities of twin or triplet drugs for novel medicines. The ester or amide moieties, which are included in most of reported twin drugs, are potentially labile to enzymatic and chemical hydrolysis of the moieties, although stability of compound MCL-144 (13b) for hydrolysis was estimated. The ester functionality was a crucial structure for elicitation of binding affinities in twin drugs with two butorphan (7) pharmacophores. Therefore, for interpretation of the pharmacological results obtained using twin drugs with ester or amide moieties, it may be important to take metabolites derived from hydrolysis into account. In general,

270

H. Fujii

N OH MeO O OMe HO

O

N N O

OH

N O O

OMe N

OMe 104

105

Fig. 21 Structures of expected triplet drug 104 and compound 105

twin and triplet drugs have high molecular weights, many functional groups, and many rotatable bonds. These properties seem to be inadequate from the viewpoints of drug-likeness [163] and blood-brain barrier permeability [164]. However, it is noteworthy that many compounds showed their pharmacological effects not only by i.c.v or i.t administration but also by systemic routes (i.p., i.v., i.m., or s.c. routes). Some twin drugs were utilized to investigate receptor dimerization. Based on the results obtained using twin drugs KDN-21 (37a) with d and k antagonist pharmacophores and KDAN-18 (44a) with d antagonist and k1 agonist pharmacophores, the d – k heterodimer is proposed to correspond to d1 and k2 subtypes while the d2 and k1 subtypes would correspond to d and k homodimers, respectively. There are both reports for and against this conclusion [12, 80, 84, 165], and further investigations would be required to obtain definitive agreement. Twin drugs showing significant effects can be divided into two categories by the spacer length: those with longer chains (at least ten atom units although the length depended on the structure of the pharmacophores and/or the linked positions) and those with shorter linking spacers. It is believed that the twin drugs with longer and appropriate length spacers can bind two active sites simultaneously to exert their effects. On the other hand, although the twin drugs with shorter linker do not seem to bridge two active sites, their binding affinities and/ or potencies of antinociception are greater. Compounds in this category may interact with their target molecules by another mode of action. A representative peptidic twin drug, biphalin (106) (Tyr-D-Ala-Gly-Phe-NH-)2 [166], belongs to this category and showed more potent and longer lasting analgesia than the monomeric peptide. Why biphalin (106) exhibited such pharmacological profiles remains poorly understood. Elucidation of the mode of action of such compounds with shorter spacers may lead to innovation of novel concepts in drug design.


271

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Twin and Triplet Drugs in Opioid Research 71. 72. 73. 74. 75. 76. 77. 78. 79. 80. 81. 82. 83. 84. 85. 86. 87. 88. 89. 90. 91. 92. 93. 94. 95. 96. 97. 98. 99. 100. 101. 102. 103. 104. 105. 106. 107. 108.

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Top Curr Chem (2011) 299: 277–307 DOI: 10.1007/128_2010_84 # Springer-Verlag Berlin Heidelberg 2010 Published online: 31 July 2010

3D-Pharmacophore Identification for k-Opioid Agonists Using Ligand-Based Drug-Design Techniques Noriyuki Yamaotsu and Shuichi Hirono

Abstract A selective k-opioid receptor (KOR) agonist might act as a powerful analgesic without the side effects of m-opioid receptor-selective drugs such as morphine. The eight classes of known KOR agonists have different chemical structures, making it difficult to construct a pharmacophore model that takes them all into account. Here, we summarize previous efforts to identify the pharmacophore for k-opioid agonists and propose a new three-dimensional pharmacophore model that encompasses the k-activities of all classes. This utilizes conformational sampling of agonists by high-temperature molecular dynamics and pharmacophore extraction through a series of molecular superpositions. Keywords k-Opioid receptor Agonist Molecular superposition Pharmacophore Selectivity

Contents 1 2

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 278 Skeleton Classes for KOR Agonists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 279 2.1 Peptides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 280 2.2 Benzomorphans . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 280 2.3 Morphinans . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 281 2.4 Arylacetamides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 281 2.5 Diazabicyclononanones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 282 2.6 Bicyclic Guanidines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 283 2.7 Benzodiazepines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 283 2.8 Neoclerodane Diterpenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 284

N. Yamaotsu ð*Þ and S. Hirono School of Pharmacy, Kitasato University, 5-9-1 Shirokane, Minato-ku, Tokyo 108-8641, Japan e-mail: [email protected]

278

N. Yamaotsu and S. Hirono

3

Previous Efforts to Identify the Pharmacophore for KOR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 284 3.1 Schematic Models Based on SAR Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 285 3.2 Ligand-Based Models Using Molecular Superposing Methods . . . . . . . . . . . . . . . . . . . . . 286 3.3 Docking Models Based on Homology Modeling and Ligand Docking Methods . . . 287 4 Universal Pharmacophore Model Accounting for All Known Skeletons of k-Agonists . . 290 4.1 Preparation of Data Sets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 290 4.2 Conformational Sampling of k-Agonists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 290 4.3 Pharmacophore Extraction Through a Series of Molecular Superpositions . . . . . . . . . 293 4.4 Determined 3D-Pharmacophore for All k-Agonists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 293 5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303

Abbreviations 2D 3D d/k m/k BCG CCK DOR GPCR Ki KOR MD MMFF MOR ORL1 pKa QSAR SAR

Two-dimensional Three-dimensional Ki(d)/Ki(k) Ki(m)/Ki(k) Bicyclic guanidine Cholecystokinin d-Opioid receptor G-protein-coupled receptor Inhibition constant k-Opioid receptor Molecular dynamics Merck molecular force field m-Opioid receptor Opioid receptor-like 1 receptor Negative logarithm of acid dissociation constant Quantitative structure–activity relationship Structure–activity relationship

1 Introduction Opioid receptors belong to the superfamily of G-protein-coupled receptors (GPCRs), and interact with morphine (Fig. 1) and related opiate alkaloids, as well as the various endogenous opioid peptides [1, 2]. They are divided into at least three types: m-opioid receptors (MORs), k-opioid receptors (KORs), and d-opioid receptors (DORs) [2]. A selective KOR agonist could be a powerful analgesic drug without the clinically limiting side effects (such as dependence, respiratory depression, and constipation) of MOR-selective drugs like morphine [3, 4].

3D-Pharmacophore Identification for k-Opioid Agonists Fig. 1 Morphine rings and positions

279 H+ N

9

16

D 15

10

B

11

H

8 7

14 C 13

12 A

1

OH

Morphine

3 2

6

5 E O 4 OH

The known KOR agonists belong to the following eight classes: peptides (such as dynorphin A) [5–8]; benzomorphan derivatives (such as bremazocine) [9–13]; morphinan derivatives (such as TRK-820) [14–16]; arylacetamide derivatives (such as U-50488H) [17–22]; diazabicyclononanone derivatives (such as HZ2) [23–25]; bicyclic guanidine (BCG) derivatives (such as TPI 614-1) [8, 26, 27]; benzodiazepine derivatives (such as tifluadom) [28–31]; and neoclerodane diterpene derivatives (such as salvinorin A) [32–35]. Because these classes have different chemical structures, many computational studies have attempted to determine the pharmacophore for k-selective agonists. These studies have been divided into ligand-based methods [16, 36–38] and docking-based methods [7, 13, 29, 31–36, 39–42]. With the former approach, it is difficult to superpose different skeletons onto one another, so ligand-based methods have been applied to congeneric compounds or the charged NH+ moiety corresponding to the amino (N)-terminus of peptides. The latter approach is powerful when the crystal structure of a target protein is known [43]; however, X-ray structures of opioid receptors are not available. All docking studies of opioid ligands have therefore been based on homology models derived from other GPCRs. Homology modeling is a well-established method, and the main chain of a target protein is predictable [44, 45]. However, many problems remain to be overcome prior to docking, including the induced fit and the difference between agonist/antagonist binding modes [45–47]. Hence, different docking protocols against KOR tend to result in different models [32–35]. Here, we propose a new ligand-based model that can account for the k-activities of all classes, and utilizes conformational sampling of agonists by high-temperature molecular dynamics (MD) and pharmacophore extraction through a series of molecular superpositions.

2 Skeleton Classes for KOR Agonists We surveyed the features of each of the eight KOR agonist skeleton classes.

280

a b


Dynorphin A

+H N-Try1-Gly2-Gly3-Phe4-Leu5-Arg6-Arg7-IIe8-Arg9-Pro10-Lys11 3 -Leu12-Lys-13-Trp14-Asp15-Asn16-Gln17-COO–

Dynorphin B

+H

1 2 3 4 5 6 7 8 9 10 11 3N-Try -Gly -Gly -Phe -Leu -Arg -Arg -Gln -Phe -Lys -Val -Val12-Thr13-Arg14-Ser15-Gln16-Glu17-Asp18-Pro19-Asn20-Ala21-Tyr22 -Glu23-Glu24-Leu25-Phe26-Asp27-Val28-COO–

Fig. 2 Sequences of (a) dynorphin A and (b) dynorphin B

2.1

Peptides

All endogenous m-opioid, k-opioid, and d-opioid peptides have +H3N-Tyr1-Gly2Gly3-Phe4 as the common message domain [2]. The endogenous agonists of KORs are dynorphins (Fig. 2). Dynorphin A has high affinity, with a binding constant Ki(k) of 0.17–0.5 nM and a k-selectivity of Ki(m)/Ki(k) = m/k = 64 [5, 48, 49]. Various structural modifications have been made to dynorphin molecules in attempts to identify the relevant pharmacophore. Chavkin and Goldstein carried out successive removal of carboxyl (C)-terminal amino acids from dynorphin A-(1-13) [5]. Their experiment indicated important contributions of Arg7, Lys11, and Lys13 to k-activity. Moreover, the removal of Tyr1 led to loss of biological activity. Lapalu et al. studied chimeric peptides between dynorphin A and nociceptin, which is the endogenous agonist of the opioid receptor-like 1 (ORL1) receptor [49], and found that Leu5-Arg6 of dynorphin A is important for k-activity. Thus, Leu5-Arg6-Arg7 is thought to be the address domain responsible for k-specificity. In an attempt to obtain shorter peptides using combinatorial libraries, Houghten and colleagues identified the highly selective tetrapeptide, D-Phe-D-Phe-D-Nle-DArg-NH2 (Ki(k) = 1 nM, m/k > 10,000) [6, 8]. Przydzial et al. studied cyclic tetrapeptides and obtained MP-16 (Tyr-C[D-Cys-Phe-D-Cys]-NH2) with an intermediate binding affinity (Ki(k) = 38.7 nM) [7].

2.2

Benzomorphans

Benzomorphans lack the C ring of morphine (Fig. 3). The k-activities of benzomorphans were originally identified by the unique in vivo pattern of ketazocine (ketocyclazocine) agonist activity [2], so ketazocine is the first synthetic k-selective agonist. Other benzomorphans include bremazocine, MPCB, pentazocine, and eptazocine [9–13]. These were synthesized in an effort to maximize k-selectivity and to minimize morphine-like side effects. For example, the selectivity of bremazocine is m/k ¼ 3.8–15 [12, 20, 50]. Bremazocine, pentazocine, and eptazocine are KOR agonists and MOR antagonists [9, 10, 51], while pentazocine and eptazocine are drugs that are used clinically.

3D-Pharmacophore Identification for k-Opioid Agonists

a

b

c HO

H+ N

281 O

H+ N

O

H+ N

O OH

OH

OH cis-(−)-MPCB

(−)-Bremazocine

Ketazocine

d

e H+ N

NH+

OH Pentazocine

OH (−)-Eptazocine

Fig. 3 Chemical structures of benzomorphans: (a) ketazocine, (b) bremazocine, (c) MPCB, (d) pentazocine, and (e) eptazocine

2.3

Morphinans

Morphine is classified as a 4,5-epoxymorphinan but a m-selective agonist (m/k ¼ 0.02–0.08) [31, 50]. However, butorphanol, which belongs to the morphinan group (Fig. 4), induces analgesia by acting predominantly at the KOR. The novel k-selective agonist TRK-820 (m/k ¼ 2.6) is also a 4,5-epoxymorphinan, but has sedative activity with no adverse effects [14, 16]. The C ring of morphine is cyclohexene, while that of butorphanol and TRK-820 is cyclohexane. Moreover, TRK-820 has a side chain corresponding to the address domain at the 6-position. Butorphanol and TRK-820 (nalfurafine) are approved drugs. NS-22 and KNT-63 are known to be TRK-820-related compounds. NS-22 is a 6,14-epoxymorphinan with equal m- and k-affinities [15, 16]. KNT-63 is a 4,5epoxymorphinan with an oxabicyclo[2.2.2]octane moiety instead of a cyclohexane moiety of the C ring [16]. It is interesting to note that the chirality of the KNT-62 (7R) side chain is opposite to that of KNT-63 (7S). The selectivities of KNT-63 and KNT-62 are 1.9 and 1.3, respectively [16].

2.4

Arylacetamides

The first arylacetamide to be identified, U-50488H, showed very high k-selectivity (m/k ¼ 630–845) [17, 20, 50]. Its skeleton is different from that of morphine, and

282


a

H+ N

b

OH

c

OH

H+ N

H+ N

O O

H N

O

N

O

O OH

OH Butorphanol

OH

TRK-820

NS-22

d

e O

O

H+ N

H+ N

N H

O

N H

O

O

O

OH

OH

KNT-63

KNT-62

Fig. 4 Chemical structures of morphinans: (a) butorphanol, (b) TRK-820, (c) NS-22, (d) KNT-63, and (e) KNT-62

a

b O N

c

Cl

O

O

O

N

Cl

NH+

O

N

NH+ U-50488H

Cl Cl

NH+ U-69593

DuP 747

Fig. 5 Chemical structures of arylacetamides: (a) U-50488H, (b) U-69593, and (c) DuP 747

is comparatively simple (Fig. 5). Benzeneacetamido-piperizines have a related skeleton [52]. The identification of U-50488H was followed by compounds such as U-69593, DuP 747, PD117302, and CI-977 [18–22]. Arylacetamides do not produce morphine-like side effects such as respiratory depression and constipation. However, they do possess dysphoric and psychotomimetic properties which are thought to be common to k-agonists [3].

2.5

Diazabicyclononanones

The diazabicyclononanone (3,7-diazabicyclo[3.3.1]nonan-9-one) skeleton is placed in a relatively new class [23–25], having two positively charged groups, which is also common to peptides (Fig. 6). Diazabicyclononanones include HZ1, 3FLB, and HZ2 [23, 53], the last of which exhibits the next highest k-selectivity (m/k > 66.7) after


a

283

b

c

NH+ O

O

NH+ O

O

O O

N

N H2+

O

O

O

NH+ O O

O O

O

O

H+ N N

N

H+ N N F

HZ1

HZ2

F 3FLB

Fig. 6 Chemical structures of diazabicyclononanones: (a) HZ1, (b) HZ2, and (c) 3FLB

N N H+

O N

TPI 614-1

Fig. 7 Chemical structure of a bicyclic guanidine, TPI 614-1

arylacetamides [23]. HZ2 also showed a strong and long-lasting antinociceptive effect in mouse tail-flick tests. In contrast to HZ1 and 3FLB, HZ2 offers a high oral availability in different animal species [23, 53].

2.6

Bicyclic Guanidines

Houghten and colleagues obtained a new trisubstituted bicyclic guanidine (BCG) skeleton following cyclization of reduced N-acylated dipeptides [54]. The BCGs were selected after screening of a positional scanning-synthetic combinatorial library, and were evaluated in a KOR-binding assay [8, 26, 27]. BCGs show intermediate-to-low binding affinities for KOR, with the best compound, TPI 614-1 (Fig. 7), having a k-affinity of 39 nM [8, 27]. Selectivity and agonist activity have not yet been reported.

2.7

Benzodiazepines

Tifluadom is a 2-[(acylamino)methyl]-1,4-benzodiazepine (Fig. 8), with k-agonist and cholecystokinin-A (CCK-A) antagonist abilities [29]. Cappelli and colleagues successfully discriminated KORs from CCKs, and KORs from MORs using the 2-[(acylamino)ethyl] derivatives [29, 31]. Although tifluadom is a racemic

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Fig. 8 Chemical structure of a benzodiazepine, tifluadom

O N H

N

S

N F (+)-(2S)-Tifluadom

O

Fig. 9 Chemical structure of a neoclerodane diterpene, salvinorin A O O

H

H

H O O

O Salvinorin A O

O

mixture, both enantiomers can bind to the KOR. The affinity and selectivity of (+)(2S)-tifluadom are Ki(k) ¼ 0.17 nM and m/k ¼ 8.8, and those of ()-(2R) isomer are Ki(k) ¼ 0.71 nM and m/k ¼ 35 [30]. However, (+)-(2S) isomer induces KOR-related reactions in vivo, whereas ()-(2R) is related to the benzodiazepine receptor [28].

2.8

Neoclerodane Diterpenes

A neoclerodane diterpene skeleton for the KOR was recently elucidated and shown to be notably different as it does not possess a positively charged group. Salvinorin A is an example of a neoclerodane diterpene isolated from Salvia divinorum (Fig. 9), which shows very high k-selectivity (m/k > 526) [32, 55]. Salvinorin A is a KOR agonist both in vivo and in vivo [53, 56], but demonstrates little anti-scratching activity and no antinociception in mice. It has been reported as a hallucinogenic, and Harding and colleagues were able to convert it into a new m-agonist, herkinorin, using chemical modifications [55, 57]. Thus, it is possible for a neoclerodane diterpene skeleton to be a template for other opioid agonists.

3 Previous Efforts to Identify the Pharmacophore for KOR Biochemists, medicinal chemists, and computer chemists have previously tried to identify the pharmacophore for KOR. Their efforts include the following: schematic models based on structure–activity relationship (SAR) studies; ligand-based


285

models with molecular superposing; and docking models based on homology modeling and ligand docking. Here, we outline the history and summarize the results.

3.1

Schematic Models Based on SAR Studies

Studies of endogenous dynorphins clarified that +H3N-Tyr1-Gly2-Gly3-Phe4 is the common message domain for all m-opioid, k-opioid, and d-opioid peptides, whereas Leu5-Arg6-Arg7 is the address domain responsible for k-specificity [5, 49]. In studies of the synthetic k-selective antagonists norbinaltorphimine (norBNI) and 50 -guanidinonaltrindole (GNTI) (Fig. 10), a second positively charged group was considered to be the address domain of morphinans [58–60]. However, all selective k-agonists, with the exception of peptides and diazabicyclononanones, lack a second positively charged group, while neoclerodane diterpenes lack any positively charged group. The first model for KOR was proposed by Martin and consisted of an anionic subsite corresponding to the N-terminal charged NH+ moiety of the peptide agonist, the benzene subsite for Tyr, and the ketonic oxygen site for ketazocine (at the 10position of morphine) [61]. Feinberg et al. attempted to explain differences between agonists and antagonists at opioid receptors [62], and outlined a model composed of the amine binding subsite, the lipophilic subsite for Tyr, the agonist binding subsite corresponding to the address domain, and the specific antagonist binding subsite corresponding to large side chains at the 17-position of morphine. With the conversion of binding modes, Protoghese et al. observed substitutioneffect differences on the same positions between morphine and the related skeletons, phenylpiperidines and allylprodines, with the MOR [63, 64]. They suggested that different skeletons might adopt different binding modes. According to this model, the binding site of opioid receptors might consist of the anionic (A) subsite, the Tyr (T) subsite, and the Phe (P) subsite (Fig. 11). The binding mode of phenylpiperidines and allylprodines is thought to be the A–P orientation, whereas that of morphine is the A–T orientation. However, these models only took into account morphine-related skeletons such as benzomorphans and morphinans for the KOR.

a

b H+ N

OH

OH

H+ N

H+ N

N O HO

H N

OH

N O H

OH HO

OH

norBNI

GNTI

Fig. 10 Structures of k-antagonists: (a) norBNI and (b) GNTI

NH2+ NH2

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Fig. 11 Schematic models proposed by Protoghese et al. [64]: (a) peptide, (b) morphine, and (c) allylprodine. The anionic (A) subsite interacts with the charged N-terminus of the opioid peptide. The T and P subsites recognize the aromatic residues of Tyr1 and Phe4 of the opioid peptide. Reprinted from [64] with permission from American Chemical Society. Copyright (1981)

In addition, traditional quantitative structure–activity relationship (QSAR) models were reported. Gozalbes et al. attempted to predict the blood–brain barrier permeabilities of four arylacetamides using linear discriminant analysis [65], while Medina-Franco et al. discriminated between active and inactive BCG compounds using two-dimensional (2D) and three-dimensional (3D) structuralsimilarity methods [66].

3.2

Ligand-Based Models Using Molecular Superposing Methods

It is difficult to superpose different skeletons onto one another. Therefore, these methods have required congeneric compounds or the charged NH+ moiety as a matching point. Lavecchia et al. constructed a ligand-based model for eight arylacetamides in which the charged NH+, amide carbonyl, and benzene ring moieties were used as overlapping points [36]. The reported distances among the pharmacophoric points, and the root-mean-square distances from the reference arylacetamide U-50488H, were relatively small. Filizola et al. proposed each d, m, and k-pharmacophore model using morphinans, fentanyl, and so on [37, 38]. Their k-model was made up of one k-specific moiety and five common recognition moieties: the charged NH+ moiety, two hydrophobic moieties of the C and D-rings (corresponding to the 8 and 15-positions of morphine), the aromatic ring, and the phenolic hydroxyl group. The one k-specific hydrophobic moiety is a long side chain at the positive amine. In our previous study using the SUPERPOSE program, which does not require an anchor point, the arylacetamide U-50488H was superposed on the morphinan TRK-820 [16]. This model indicated a basic amine and an amide side chain as overlapping points (Fig. 12).


287

Fig. 12 Our previous superimposition of U-50488H (arylacetamide) on TRK-820 (morphinan) [16]. TRK-820 is shown in green, and U-50488H is shown in white. Sphere colors indicate the following properties (see Sect. 4.3): hydrophobic (HP, white); aromatic (AR, green); hydrogenbond donors (HD, blue); hydrogen-bond acceptors (HA, red); and hydrogen-bond donors/acceptor ˚ , respectively. The TRK-820 (DA, violet). Large and small spheres represent radii of 1 and 0.5 A and U-50488H spheres are represented by dotted and solid spheres, respectively. Only matched spheres are indicated

3.3

Docking Models Based on Homology Modeling and Ligand Docking Methods

As the X-ray structures of opioid receptors are not yet available, all docking models of k-agonists have been based on homology models derived from other GPCRs. Subramanian et al. docked arylacetamides to a homology model of KOR, and showed that a salt bridge between the amino proton of the ligands and Asp138, which is the conserved residue in opioid receptors, is important for the agonist association [39]. Lavecchia et al. reported docking models of arylacetamides and a benzomorphan (Fig. 13), and also found that the salt bridge with Asp138 is important for the ligand-receptor recognition process [36]. A hydrogen bond between the amide oxygen of the arylacetamide and the hydroxyl group of Tyr312, and a p-stacking between the dichlorobenzene ring of the arylacetamide and the phenol ring of Try312 were observed, while a hydrogen bond between the phenolic hydroxyl group of the benzomorphan and the imidazole ring of His291 was indicated. In a docking model of benzodiazepines reported by Anzini and colleagues, these residues were also indicated as key [29, 31]. Kane et al. reported docking models of an arylacetamide, the analog of DuP 747, and listed Asp138, His291, and Tyr312 as important residues [42]. Pogozheva et al. docked the cyclic peptide MP-16, and

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Fig. 13 Docking models of an arylacetamide, the derivative of DuP 747 (yellow), and a benzomorphan, MPCB (cyan), reported by Lavecchia et al. [36]. Reprinted from [36] with permission from American Chemical Society. Copyright (2000)

showed that the N-terminus, the hydroxyl group of Tyr, and the benzene ring of Phe in the peptide interacted with Asp138, His291, and Tyr312 on the KOR, respectively [41]. In addition, both Pogozheva et al. [41] and Kane et al. [42] docked antagonists norBNI and GNTI, showing that the second positive charge corresponding to the address domain interacted with Glu297. In the model reported by Holzgrabe and Brandt, in which HZ2 was docked, the two charged NH+ groups interacted with Glu209 and Glu297 instead of the conserved Asp138. A pstacking was also observed between Tyr312 and the pyridine ring of HZ2, while a covalent bond between the side chain of Lys200 and the ketocarbonyl carbon atom of HZ2 was predicted, resulting in the long duration of action [40]. Two different docking models have been proposed for salvinorin A. The first (Fig. 14a), put forward by Roth and colleagues, involves the furan ring of salvinorin A interacting with Tyr119 and Tyr320 by a branched hydrogen bond, and a CH-p interaction between the acetyl group of salvinorin A and Tyr313 [32, 33]. In the second model (Fig. 14b), put forward by Kane et al., only Tyr320 is stacked with the furan ring. A CH-p interaction is predicted between Tyr119 and the acetyl group, with another interaction predicted between Tyr313 and the methyl ester group [34, 35]. As described above, docking models strongly depend on homology models of KORs and docking protocols. The state-of-the-art in modeling and ligand docking for GPCRs, to which opioid receptors belong, will be given by [44–47].


289

Fig. 14 Two different docking models for salvinorin A, reported by Roth and colleagues [33] (a) and Kane et al. [35] (b). Reprinted from [33] [Copyright (2005)] and [35] [Copyright (2008)] with permission from American Chemical Society

290


4 Universal Pharmacophore Model Accounting for All Known Skeletons of k-Agonists A universal pharmacophore model taking into account all skeletons remains elusive, because of the differences in their chemical structures. We therefore tried to construct such a 3D-pharmacophore model using the superposing method, which does not require a predefined anchor point [67].

4.1

Preparation of Data Sets

In order to identify the 3D-pharmacophore for k-agonists, we prepared two data sets: the training set (Fig. 15) and the test set (Fig. 16). Ligands with high affinities and high to intermediate selectivities were used as the training set, which consisted of four morphinans (TRK-820, KNT-63, KNT-62, and NS-22) [14–16], two arylacetamides (U-69593 and U-50488H) [17, 18], and one neoclerodane diterpene (salvinorin A) [32]. In the test set, the affinities ranged from high to intermediate, and the selectivities ranged from high to low. The test set comprised two morphinans (TAN-821 [68] and morphine), one benzomorphan (bremazocine) [9, 12], one diazabicyclononanone (HZ2) [23, 24, 69], one BCG (TPI 614-1) [8, 26, 27], one benzodiazepine (tifluadom) [28–31], and one peptide (MP-16) [7]. Binding assays of the following compounds (TRK-820, KNT-63, KNT-62, NS-22, TAN-821, morphine, and U-69593) were carried out in duplicate using guinea pig brain homogenates (the cerebellum for k and the forebrain for m). The ability of each compound was evaluated by displacement of [3H]-U-69593 (k) and [3H]-DAMGO (m). Data for other compounds were taken from [7, 16, 20, 23, 24, 27, 30–32, 50, 55]. All agonists were prepared by SYBYL 6.9.1 (Tripos Inc.). The ionization states of molecules were determined according to references or pKa calculations (ADMET Predictor 4.0, Simulations Plus, Inc.).

4.2

Conformational Sampling of k-Agonists

Conformational analyses of all compounds were performed on an Apple Power Mac G5 (PowerPC G5; 2.5 GHz; two central processing units [CPUs]) using the Conformational Analyzer with Molecular Dynamics And Sampling (CAMDAS) 2.1 program [70]. Ten MD calculations were simultaneously performed using different initial structures. Each of the MD calculations was carried for 1 ns with an integral time step of 1 fs. The lengths of the covalent bonds were fixed. The temperature of the system was maintained at 1,200 K in order to enhance the sampling efficiency. Conformers were sampled at every 100 steps, and then each conformer was minimized until the root-mean-square of the gradients of the


291

Training Set O

Morphinans OH

O

N

N H OH

O

NH+

NH+

O O

O

TRK-820 Ki(κ) = 0.195~0.225 nM OH (μ / κ = 2.6)

OH

KNT-63 Ki(κ) = 0.111 nM (μ / κ = 1.9)

O N H OH

O NH+

O NH+

H N

O

O KNT-62 OH Ki(κ) = 0.152 nM (μ / κ = 1.3)

NS-22 OH Ki(κ) = 0.135 nM (μ / κ = 1.0)

Arylacetamides O

O

O

N NH+

N NH+

U-69593 Ki(κ) = 0.475~1.89 nM (μ / κ = 292~3672)

O

Cl U-50488H Ki(κ) = 0.69~0.70 nM (μ / κ = 630~845)

O

Neoclerodane Diterpene

O

Cl

H

H

H O O

O

Salvinorin A O

O

Ki(κ) = 1.9~16 nM (μ / κ > 526)

Fig. 15 k-Agonists in training set and their property spheres. Training set comprising four morphinans (TRK-820, KNT-63, KNT-62, and NS-22), two arylacetamides (U-69593 and U50488H), and one neoclerodane diterpene (salvinorin A). The spheres on each molecule are property spheres for molecular superposition. The colors of the spheres indicate the following properties: hydrophobic (HP, black); aromatic (AR, green); hydrogen-bond donors (HD, blue); hydrogen-bond acceptors (HA, red); and hydrogen-bond donors/acceptor (DA, violet). Large and ˚ , respectively. m/k ¼ Ki(m)/Ki(k) small spheres indicate radii of 1 and 0.5 A

292


Test Set Morphinans

O N

NH+

NH+

OH

OH O

O TAN-821 Ki(κ) = 0.279 nM

OH

OH

(μ / κ = 1.2)

Benzomorphan

Morphine Ki(κ) = 33.7~151 nM (μ / κ = 0.02~0.08)

Diazabicyclononanone

HO NH+

+HN

O O

O O

O +HN

(−)-Bremazocine

N

OH Ki(κ) = 0.04~0.12 nM

HZ2

N

Ki(κ) = 15 nM (μ / κ > 66.7)

(μ / κ = 3.8~15.0) Benzodiazepine

Bicyclic Guanidine

H N

N N H+

N

O

O S

N N F TPI 614-1

(+)-(2S)-Tifluadom

Ki(κ) = 39 nM (μ / κ = n.d.*)

Ki(κ) = 0.17 nM (μ / κ = 8.8)

Peptide

S O

+H

3N

CH2

S O

CH2

H H H H H C C N C C N C C N C C NH2 H H CH2 O O CH2

MP-16 (Tyr-C[D-Cys-Phe-D-Cys]-NH2) OH

Ki(κ) = 38.7 nM (μ / κ = 0.03)

Fig. 16 k-Agonists in test set and their property spheres. Test set comprising two morphinans (TAN-821 and morphine), one benzomorphan (bremazocine), one diazabicyclononanone (HZ2),


293

˚ –1. All conformations were cluspotential energy was below 0.001 kcal mol–1 A tered with torsion angles of heavy atoms using a threshold of 30 . The Merck molecular force field (MMFF) 94s was used to evaluate the potential energy surface of the molecule [71, 72]. The dielectric constant was 80, but no cut-off distance for non-bonded interactions was used. The conformers within 15 kcal mol1 of the minimum energy of the molecule were adopted for superposition because of the low probability of the existence of conformers with higher energies.

4.3

Pharmacophore Extraction Through a Series of Molecular Superpositions

All agonist molecule conformations were superposed onto each another using the parallel version of the SUPERPOSE program [73]. This superposes two molecules based on the physicochemical properties of the atomic groups, which is useful for elucidating the 3D-pharmacophore and estimating the binding conformation of each molecule by distinguishing it from the many conformations that are generated by CAMDAS. The program considers five types of physicochemical property: hydrophobic (HP), aromatic (AR), hydrogen-bond donors (HD), hydrogen-bond acceptors (HA), and hydrogen-bond donors/acceptors (DA). Each type is repre˚ or 0.5 A ˚ ) and is assigned to a sented as a sphere with a predefined radius (1.0 A functional group in a molecule (Fig. 15). After molecular superposition, the overlaps of the spheres are scored using the scoring matrix (Table 1). One set of agonist conformations was chosen for the binding conformations based on the sum of the scores, and the 3D-pharmacophore was represented as one set of matched spheres on the binding conformations. For the training set we used a series of superpositions to determine the binding conformations. The binding conformations of the test set were extracted using those of the training set. The superposition calculations were performed using 28 nodes of a Dell PowerEdge 1950III (Quad Core Xeon X5460; 3.16 GHz; 56 CPUs in total).

4.4

Determined 3D-Pharmacophore for All k-Agonists

The alignment of training set KOR agonists in estimated binding conformations and the 3D-pharmacophore of TRK-820 are shown in Fig. 17. The 3D-pharmacophore

526 – – HA HP HZ2 >66.7 HD – HA HP Bremazocine 3.8–15.0 HD – – AR Tifluadom 8.8 – AR – – TRK-820 2.6 HD AR DA HP KNT-63 1.9 HD AR DA HP KNT-62 1.3 HD AR DA HP TAN-821 1.2 HD HP – HP NS-22 1.0 HD AR DA HP Morphine 0.02–0.08 HD HP – AR MP-16 0.03 HD AR DA AR TPI 614-1 n.d.b HD AR HA HP a m/k ¼ Ki(m)/Ki(k) b No data

Fig. 21 The effect of bridging on k-affinity of the d-agonist TAN-67. (a) TAN-67 (Ki(k) ¼ 732.45 nM, m/k ¼ 0.388, d/k = 0.197 102). (b) The 10-methylene-bridged SN-28 (Ki(k) ¼ 10.2 nM, m/ k ¼ 1.13, d/k = 1.4 102)

a

H+ N

b

H

N

HA2 – – HA – DA HA HA HA HA HA HA DA HA –

H+ N

HP3 HP – HP – HP HP HP HP HP HP – HP – –

HA3 HA HA HA HA – HA HA HA HA DA – DA – –

H

N

TAN-67 OH

SN-28 OH

m-receptor (see Sect. 3.1 and Fig. 11) [63, 64]. The m-binding mode of these ligands is thought to be the A–P orientation (type III), whereas that of morphine is the A–T orientation (type II). In our model, the k-binding mode of morphine was predicted to be type III (Fig. 24). However, the binding site of the k-receptor could not accept morphine, because it is larger than bremazocine. In addition, the conformation of the C ring of TRK-820 determined by our calculation was the boat form, which is important for the k-activity to maintain the binding conformation. By contrast, the C ring of morphine is the cyclohexene ring. In a previous study, we reported the type III orientation for arylacetamides by superimposing them on TRK-820 (Fig. 12) [16]. In the current study using all k-selective classes, the type II orientation was found to be the most likely (Fig. 23), followed by type III. However, it is possible that arylacetamides adopt both in the k-receptor. The orientation of type IV for salvinorin A in our model differs from the docking models reported by Roth and colleagues [32, 33] and by Kane et al. [34, 35] (Fig. 14). In our model, salvinorin A avoided the position of the charged NH+ group (HD1), that is, the vicinity of the conserved Asp138 in all opioid receptors,


299

Fig. 22 Superimposition of bremazocine (benzomorphan) on TRK-820 (morphinan) and their 3Dpharmacophores. TRK-820 is shown in green, and bremazocine is shown in white. The 3Dpharmacophores of TRK-820 and bremazocine are represented by dotted and solid spheres, respectively. Matched spheres are indicated by underlined labels

Fig. 23 Superimposition of U-50488H (arylacetamide) on TRK-820 (morphinan) and their 3D-pharmacophores. TRK-820 is shown in green, and U-50488H is shown in white. The 3D-pharmacophores of TRK-820 and U-50488H are represented by dotted and solid spheres, respectively. Matched spheres are indicated by underlined labels

300


Fig. 24 Superimposition of morphine (morphinan) on TRK-820 (morphinan) and their 3D-pharmacophores. TRK-820 is shown in green, and morphine is shown in white. The 3D-pharmacophores of TRK-820 and morphine are represented by dotted and solid spheres, respectively. Matched spheres are indicated by underlined labels

Fig. 25 Superimposition of salvinorin A (neoclerodane diterpene) on TRK-820 (morphinan) and their 3D-pharmacophores. TRK-820 is shown in green, and salvinorin A is shown in white. The 3D-pharmacophores of TRK-820 and salvinorin A are represented by dotted and solid spheres, respectively. Matched spheres are indicated by underlined labels


301

Fig. 26 Superimposition of tifluadom (benzodiazepine) on TRK-820 (morphinan) and their 3Dpharmacophores. TRK-820 is shown in green, and tifluadom is shown in white. The 3D-pharmacophores of TRK-820 and tifluadom are represented by dotted and solid spheres, respectively. Matched spheres are indicated by underlined labels

Fig. 27 Superimposition of HZ2 (diazabicyclononanone) on TRK-820 (morphinan) and their 3Dpharmacophores. TRK-820 is shown in green, and HZ2 is shown in white. The 3D-pharmacophores of TRK-820 and HZ2 are represented by dotted and solid spheres, respectively. Matched spheres are indicated by underlined labels

302


Fig. 28 Superimposition of TPI 614-1 (bicyclic guanidine) on TRK-820 (morphinan) and their 3D-pharmacophores. TRK-820 is shown in green, and TPI 614-1 is shown in white. The 3Dpharmacophores of TRK-820 and TPI 614-1 are represented by dotted and solid spheres, respectively. Matched spheres are indicated by underlined labels

because it only has acceptor atoms (Fig. 25). Moreover, the furan ring of salvinorin A overlapped with that of TRK-820. It was therefore assumed that our model was reliable. Similarly, the docking model of tifluadom reported by Anzini and colleagues differed from our model [29, 31]. In the latter, the position of HD1 was not occupied by tifluadom because the imine nitrogen atom was not thought to be protonated based on our acid dissociation constant (pKa) calculation (Fig. 26). As the thiophene ring of tifluadom in our model was matched to the furan ring of TRK-820, we considered our model to be acceptable. Holzgrabe and Brandt proposed a docking model of HZ2, in which the two charged NH+ groups interacted with Glu209 and Glu297 instead of the conserved Asp138 [40]. In our model, one of the two NH+ groups occupied the HD1 position corresponding to Asp138 and the binding orientation was type II as in arylacetamides (Fig. 27). This can explain the high k-selectivity of HZ2. The k-selectivities and k-agonist activities of BCGs are unknown. In our proposed model for BCGs, the T-shape orientation of TPI 614-1 was similar to that of TRK-820, except for the amide oxygen of HA2 (Fig. 28). Therefore, BCGs might be promising as k-selective agonists.


303

5 Conclusion In conclusion, we constructed a 3D-pharmacophore model for k-agonists which can account for all known skeletons. This model can provide a rationale for the activities of existing k-agonists and for designing new k-selective agonists. Acknowledgments We reused some content from [67] with permission from Elsevier. Copyright (2010).

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.

Index

A Acrylonitrile substructures, 71 14-Acylamino-17cyclopropylmethylnormorphinone, 100 14-Acylaminocodeinones, 98, 101 14-Acylaminomorphinones, 100 Adenosine, 265 Agmatine, 259 Agonists, 121, 277, 290 Alfentanil, 106 14-Alkoxymorphinans, 63 – 6-amino acid-substituted, 80 14-Alkoxymorphinan-6-ones, 67, 71, 73 14-Alkylaminocodeinones, 98, 101 14-Alkylamino-17cyclopropylaminonormorphinone, 100 14-Alkylaminomorphinones, 100 Allylprodines, 285 Alvimopan, 32 Amines, tetracyclic, 10 14-Aminocodeinone, 93, 96, 98 Aminodihydrocodeinone, 97 14-Aminomorphindole, pharmacology, 114 14-Aminomorphinones, 96, 108 Antagonists, 121 – peripheral, 31 Antinociception, 253 Arylacetamides, 279, 281, 287

B Benzimidazole isothiocyanate (BIT), 125

Benzodiazepines, 283, 287 Benzofuromorphinans, 70, 71 – 14-alkoxy-substituted, 70, 82 Benzomorphans, 227, 280 Benzotriazole, 134 Benzotriazolylamine, 134 7-Benzylidenenaltrexone (BNTX), 34 Bicyclic guanidines (BCG), 282 Biphalin, 270 Blood–brain barrier (BBB), 32, 73, 286 Bremazocine, 253, 279, 280, 292, 297 14-Bromocodeinone dimethyl acetale, 67 4-Bromo-2,5-dimethoxyamphetamine (DOB), 144 BU224, 259 Buprenorphine, 63, 66, 105, 192 Buprenorphine-like opioid, 103 Butorphanol, 281 Butorphans, 243, 269 BW373U86 105, 121, 124

C Chiral synthesis, 1, 15 Cholecystokinin (CCK), 262, 283 14-Cinnamoylamino-17cyclopropylmethyl7,8-dihydronormorphinones, 103 14-Cinnamoylaminodihydrocodeinones, 106 Cinnamoylaminomorphinone, 103 Cisapride, 265 Clinical development, 29 Clocinnamox, 104

309

310

Clonidine, 258 Codeine, 1, 2, 17, 20, 36, 65 Codeinones, 2, 104 – pharmacology, 112 Cough model, 38 6-Cyanomorphinans, 79 Cyclazocine, 144 Cyclopropylmethyl dihydromorphinone, 109 Cyclopropylmethyl morphinans, 100, 200 Cyclopropylmethyl noraminomorphindole, 115 Cyclorphans, 243 Cyprodime, 66 – 14-phenylpropoxy derivatives, 78

D DAMGO, 108, 114, 125, 200, 248, 260, 266, 290 Deltorphin II, 253 3-Deoxyclocinnamox, 107 17-Deoxysalvinorin A, 178 Dependence, 259 Diallyl-trans-2,5-dimethylpiperazine, 136 Diarylalkenyl derivatives, 129 4-Diarylaminopiperidines, 131 4-Diarylaminotropanes, 131 Diarylmethylpiperazines, 124 Diazabicyclononanones, 279, 282, 292 7,8-Didehydromorphinans, 69 7,8-Didehydromorphinan-6-ones, 72 Dihydrocodeinones, 18, 20, 22, 103 Dihydroisocodeine, 5, 20 Dihydromorphinone, 71, 102 Dihydrothebainone, 7 Dimethylpiperazine, 32 Diphenylarylpiperazines, 133 Diphenylmethylpiperazines, 121, 124 Dmt-Tic, 249 Docking models, 287 DOR (delta opioid receptors), 105, 122, 278 DPDPE, 37 DPI-2505, 123 DPI-3290, 32 Dual action, 239 DuP, 747 282 Dynorphins, 65, 144, 279, 280

Index

E 6,14-Endoethanomorphinan, 227 Endoethanotetrahydrothebaine, 190, 228 Endorphins, 65 Enkephalins, 65 – analogs, 241 Enterobactin, 266 EOR agonist, 190 Epoxymorphinans, 93, 211, 281 Epoxymorphinones, 94 Eptazocine, 280 14-Ethoxycodeinone, 67 Etorphine, 192

F Fentanyl, 31, 81, 83, 104, 107, 111, 144, 259, 265 3FLB, 144, 282

G G-protein-coupled receptors (GPCRs), 65, 144, 221, 278 Guanidines, bicyclic, 283 50 -Guanidinonaltrindole (GNTI), 285

H Hallucinogenic, 141, 144, 284 Haloperidol, 221, 266 Hemodialysis, 57 Hepatotoxicity, 163 Herkinorin, 173, 181, 284 Hydroxyaminocodeinone, 97 14-Hydroxy-7,8-dihydromorphinone (oxymorphone), 93 14-Hydroxy-5-methylcodeinone, 68 HZ1/HZ2, 282, 301

I Imidazoline, 259 Iminium salt, propellane derivatives, 210 Increment of potency, 239 Indolofuromorphinans, 14-alkoxysubstituted, 70, 82 Indolomorphinans, 70 Indomethacin, 265

Index

K KDAN-18, 254, 270 KDN-21, 253, 270 Ketazocine, 280, 285 6-Ketomorphinans, 70 KNT-62, 291 KOR (kappa opioid receptor) agonists, 105, 122, 279

L L-365, 260, 263 Levorphanol, 63 Ligand docking methods, 287 Lysergic acid diethylamide (LSD), 144

311

Naltrindole (NTI), 34, 37, 66, 71, 82, 115, 123, 125, 242, 253 – antitussive effect, 37 Neoclerodanes, 141, 149, 279, 284, 291 Nitrocinnamoylaminocodeinone, 110 Nonnarcotics, 29 Nonpeptidic agonists/antagonists, 123 Nonpeptidic delta ligands, 121 Nonpeptidic delta opioid antagonists, diarylmethylpiperazine type, 133 Norbinaltorphimine (norBNI), 66, 242, 285 Nornaltrexone, 207 20-Norsalvinorin A, 153 NS-22, 281, 291 NS-29, 257

M MCL-144, 246 MCL-450, 249 MDAN-21, 255 Message-address concept, 242 Methoclocinnamox, 103 14-Methoxymetopon, 76 Methyl barbascoate, 152 Methylnaltrexone bromide, 32 14-O-Methyloxymorphone, 73, 81 Metoclopramide, 265 Mitogen-activated protein kinase (MAPK), 253 Molecular superposition, 277, 286 MOR (mu opioid receptor), 93, 94, 105, 122 Morphinan-6-ones, 69, 71 Morphinans, 63, 281 Morphine, 1, 3, 279 – rings/positions, 279 Morphinones, 71, 93, 98 MPS9, 262

N Nalbuphine, 250 Nalfurafine, 48, 66 Naloxone, 31, 93, 95 Naltrexamines, 241, 256 Naltrexone, 93, 95, 105, 109, 187 – C16–N17 cleavage, 200 Naltriben (NTB), 71, 82, 255

O Oligomerization, 187 Opioid drugs, 29 Opioid ligands, 31 – antitussive effects, 36 Opioid receptors, 63, 122 – DOR, 105, 122 – EOR, 190 – KOR, 277 – MOR, 93, 94, 105, 122 Opioid–cholecystokinin pharmacophores, 262 Opioid–imidazoline binding site pharmacophores, 259 Opioids, 63, 187 – agonists, 32, 34, 63, 93 – antagonists, 33, 63, 93 – k-, 141 – lead compounds, 203 Opioid–substance P pharmacophores, 261 Opium poppy, 1 Ortho ester, 190 Oxabicyclo[2.2.1]heptane skeleton, 195 Oxabicyclo[2.2.2]octane skeleton, 195 Oxabicyclo[3.2.1]octane skeleton, 223 Oxazolidinone, carboxylic acid, double decarboxylation, 203 Oxycodone, 63, 66, 71, 93, 218 6-Oxymorphamines, 241 Oxymorphones, 63, 66, 71, 74, 93, 256

312

P Papaver somniferum, 2, 65 Pentazocine, 232, 280 Peptides, 37, 49, 123, 128, 258, 261, 280 Pharmacophores, 128, 239, 277 – m/d, 255 – m/k, 243 – d/k, 251 – model, k-agonists, 290 N-Phenethylpiperidone, 260 Phenylaminodihydrocodeinone, 95 Piperazine, bisarylmethyl substituted, 32 – derivatives, diphenylmethyl bridged, 132 Poppy, 1 Propellane derivatives, 210 Pruritus, 57 14-Pyridylacryloylaminodihydromorphinones, pharmacology, 112 Pyrrolooctahydroisoquinolines, 123

R Racemic synthesis, 3 Rearrangement reaction, 187 Receptor dimerization, 239 Receptors, opioids, 122 Respiratory depression, 37, 66, 75, 79, 123, 190, 278, 282

Index

SIOM, 124 SNC, 80 32, 121, 124, 133, 144, 257 Substance P, 261

T TAN-67, 33, 124 TAN-821, 190 Thebaine, 2, 8, 11, 12, 95, 215 Thioglycolamidonordihydromorphinone, 111 Tifluadom, 279, 283, 290, 302 Tolerance, 34, 76, 112, 255, 259, 262 Tramadol, 265 Transdermal delivery, 31 1,3,5-Trioxazatriquinan skeleton, nitrogen clamp, 215 Triplet drugs, 239, 266 TRK-820 (nalfurafine hydrochloride), 29, 281, 286, 299 – antipruritic k opioid agonist, 48, 57 – postoperative surgery, 56 TRK-850, metabolic defect, 42 TRK-851, 29, 34 – antitussive d opioid antagonist, 36 Tsuji-Trost coupling, 3 Twin drugs, 239, 258 – symmetrical, 257 – two opioid pharmacophores, 243

S Salvia divinorum, 144, 284 Salvinorin A, 141, 279, 284, 288 – semisynthetic modifications, 155 Selectivity, 48, 125, 128, 277 – stereoselectivity, 4 Side effects, 2, 31, 34, 48, 65, 123, 243, 267, 280

U U-50488, 34, 144, 279, 281, 286, 291 Uremic pruritus, 57

V Valinomycin, 266

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